Add sntrup761x25519-sha512 post-quantum key exchange #348

Job	Run time
build (plain linux, 1)	1m 10s
build (multi binary, 1, 1)	1m 2s
build (multi binary, dropbearmulti argv0, 1, 1)	1m 1s
build (client only, no, PROGRAMS=dbclient)	29s
build (server only, no, PROGRAMS=dropbear)	32s
build (bundled libtom, bionic , no writev(), ubuntu-20.04, --enable-bundled-libtom --enable-werro...	39s
build (linux clang, clang)	1m 21s
build (c89, -std=c89 -Wdeclaration-after-statement, 1, --enable-pam, #define DROPBEAR_SNTRUP761 0 )	1m 8s
build (macos 14, macos-14, clang, -Wno-deprecated-declarations -Wno-undef, no, no, ranlib -no_war...	27s
build (macos 15, macos-15, clang, -Wno-deprecated-declarations -Wno-undef, no, no, ranlib -no_war...	33s
build (DEBUG_TRACE, 1, --enable-pam, #define DEBUG_TRACE 5 )	1m 9s
build (nondefault options, 1, --enable-pam)	1m 6s
build (most options disabled, --disable-harden --disable-zlib --disable-openpty --disable-lastlog...	28s
build (no sha1, no, #define DROPBEAR_SHA1_HMAC 0 #define DROPBEAR_RSA_SHA1 0 #define DROPBEAR_DH_...	31s
build (pq, no plain x25519, #define DROPBEAR_CURVE25519 0 )	1m 3s
	12m 39s

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