SwiftDriver: initial work to properly handle android cross-compilation

Sources/SwiftDriver/Jobs/GenericUnixToolchain+LinkerSupport.swift

@@ -147,14 +147,24 @@ extension GenericUnixToolchain {
         }
       }
 
+      if targetInfo.sdkPath != nil {
+        for libpath in targetInfo.runtimeLibraryImportPaths {
+          commandLine.appendFlag(.L)
+          commandLine.appendPath(VirtualPath.lookup(libpath.path))
+        }
+      }
+
       if !isEmbeddedEnabled && !parsedOptions.hasArgument(.nostartfiles) {
-        let swiftrtPath = VirtualPath.lookup(targetInfo.runtimeResourcePath.path)
-          .appending(
-            components: targetTriple.platformName() ?? "",
-            String(majorArchitectureName(for: targetTriple)),
-            "swiftrt.o"
-          )
-        commandLine.appendPath(swiftrtPath)
+        let rsrc: VirtualPath
+        // Prefer the swiftrt.o runtime file from the SDK if it's specified.
+        if let sdk = targetInfo.sdkPath.flatMap({ VirtualPath.lookup($0.path) }) {
+          rsrc = sdk.appending(components: "usr", "lib", "swift")
+        } else {
+          rsrc = VirtualPath.lookup(targetInfo.runtimeResourcePath.path)
+        }
+        let platform: String = targetTriple.platformName() ?? ""
+        let architecture: String = majorArchitectureName(for: targetTriple)
+        commandLine.appendPath(rsrc.appending(components: platform, architecture, "swiftrt.o"))
       }
 
       // If we are linking statically, we need to add all
@@ -194,7 +204,12 @@ extension GenericUnixToolchain {
         commandLine.appendPath(try VirtualPath(path: opt.argument.asSingle))
       }
 
-      if let path = targetInfo.sdkPath?.path {
+      if targetTriple.environment == .android {
+        if let sysroot = try getAndroidNDKSysrootPath() {
+          commandLine.appendFlag("--sysroot")
+          commandLine.appendPath(sysroot)
+        }
+      } else if let path = targetInfo.sdkPath?.path {
         commandLine.appendFlag("--sysroot")
         commandLine.appendPath(VirtualPath.lookup(path))
       }

Sources/SwiftDriver/Toolchains/GenericUnixToolchain.swift

@@ -117,4 +117,50 @@ public final class GenericUnixToolchain: Toolchain {
     let environment = (targetTriple.environment == .android) ? "-android" : ""
     return "libclang_rt.\(sanitizer.libraryName)-\(targetTriple.archName)\(environment).a"
   }
+
+  private func getAndroidNDKHostOSSuffix() -> String? {
+#if os(Windows)
+    "windows"
+#elseif os(Linux)
+    "linux"
+#elseif os(macOS)
+    "darwin"
+#else
+    // The NDK is only available on macOS, linux and windows hosts.
+    nil
+#endif
+  }
+
+  func getAndroidNDKSysrootPath() throws -> AbsolutePath? {
+#if arch(x86_64)
+    // The NDK's sysroot should be specified in the environment.
+    guard let ndk = env["ANDROID_NDK_ROOT"],
+          let osSuffix = getAndroidNDKHostOSSuffix() else {
+      return  nil
+    }
+    var sysroot: AbsolutePath =
+        try AbsolutePath(validating: ndk)
+            .appending(components: "toolchains", "llvm", "prebuilt")
+            .appending(component: "\(osSuffix)-x86_64")
+            .appending(component: "sysroot")
+    return sysroot
+#else
+    // The NDK is only available on an x86_64 host.
+    return nil
+#endif
+  }
+
+  public func addPlatformSpecificCommonFrontendOptions(
+    commandLine: inout [Job.ArgTemplate],
+    inputs: inout [TypedVirtualPath],
+    frontendTargetInfo: FrontendTargetInfo,
+    driver: inout Driver
+  ) throws {
+    if driver.targetTriple.environment == .android {
+      if let sysroot = try getAndroidNDKSysrootPath() {
+        commandLine.appendFlag("-sysroot")
+        commandLine.appendFlag(sysroot.pathString)
+      }
+    }
+  }
 }

Sources/SwiftOptions/Options.swift

@@ -788,6 +788,7 @@ extension Option {
   public static let swiftVersion: Option = Option("-swift-version", .separate, attributes: [.frontend, .moduleInterface], metaVar: "<vers>", helpText: "Interpret input according to a specific Swift language version number")
   public static let switchCheckingInvocationThresholdEQ: Option = Option("-switch-checking-invocation-threshold=", .joined, attributes: [.helpHidden, .frontend, .noDriver])
   public static let symbolGraphMinimumAccessLevel: Option = Option("-symbol-graph-minimum-access-level", .separate, attributes: [.helpHidden, .frontend, .noInteractive, .supplementaryOutput], metaVar: "<level>", helpText: "Include symbols with this access level or more when emitting a symbol graph")
+  public static let sysroot: Option = Option("-sysroot", .separate, attributes: [.frontend, .argumentIsPath], metaVar: "<sysroot>", helpText: "Native Platform sysroot")
   public static let S: Option = Option("-S", .flag, alias: Option.emitAssembly, attributes: [.frontend, .noInteractive], group: .modes)
   public static let tabWidth: Option = Option("-tab-width", .separate, attributes: [.noInteractive, .noBatch, .indent], metaVar: "<n>", helpText: "Width of tab character.", group: .codeFormatting)
   public static let targetCpu: Option = Option("-target-cpu", .separate, attributes: [.frontend, .moduleInterface], helpText: "Generate code for a particular CPU variant")
@@ -1651,6 +1652,7 @@ extension Option {
       Option.swiftVersion,
       Option.switchCheckingInvocationThresholdEQ,
       Option.symbolGraphMinimumAccessLevel,
+      Option.sysroot,
       Option.S,
       Option.tabWidth,
       Option.targetCpu,