Remove [GT]PREL_[IS] internal binutils relocations (#199)

These have never been emitted by GCC or Clang, nor has there ever been
assembly syntax in either toolchain to create them. These are fake
internal relocations inside binutils used as part of its implementation
of linker relaxation, and so should not be part of the psABI. They
should be free to be reallocated for psABI uses (which can include
reintroducing them, if there is a need to expose them to assembly and
code generation).

Closes: #160

riscv-elf.md

@@ -575,10 +575,7 @@ Enum | ELF Reloc Type        | Description                     | Field       | C
 44   | R_RISCV_RVC_BRANCH    | PC-relative branch offset       | _CB-Type_   | S + A - P
 45   | R_RISCV_RVC_JUMP      | PC-relative jump offset         | _CJ-Type_   | S + A - P
 46   | R_RISCV_RVC_LUI       | Absolute address                | _CI-Type_   | S + A
-47   | R_RISCV_GPREL_I       | GP-relative reference           | _I-Type_    | S + A - GP
-48   | R_RISCV_GPREL_S       | GP-relative reference           | _S-Type_    | S + A - GP
-49   | R_RISCV_TPREL_I       | TP-relative TLS LE load         | _I-Type_
-50   | R_RISCV_TPREL_S       | TP-relative TLS LE store        | _S-Type_
+47-50 | *Reserved*           | Reserved for future standard use
 51   | R_RISCV_RELAX         | Instruction can be relaxed      |             |              | Paired with a normal relocation at the same address
 52   | R_RISCV_SUB6          | Local label subtraction         | _word6_     | V - S - A
 53   | R_RISCV_SET6          | Local label assignment          | _word6_     | S + A