Define PLT-generating PC-Relative data relocation

This introduces a new relocation `R_RISCV_PLT32` that follows the
existing wording for the “R_RISCV_CALL” and “R_RISCV_CALL_PLT” types,
but rather than modifying an instruction, it fills a whole, naturally
aligned, 32-bit data location with the signed byte offset from its own
location to the PLT entry for the target symbol. If the relocation is
within +/-2GB of the target symbol, the linker may relax this to a
R_RISCV_32_PCREL.

Author: PiJoules

riscv-elf.adoc

@@ -445,7 +445,9 @@ Description:: Additional information about the relocation
                                             <| S + A - P
 .2+| 58      .2+| IRELATIVE     .2+| Dynamic | _wordclass_       .2+| Relocation against a non-preemptible ifunc symbol
                                             <| `ifunc_resolver(B + A)`
-.2+| 59-191  .2+| *Reserved*                          .2+| -       |                   .2+| Reserved for future standard use
+.2+| 59      .2+| PLT32         .2+| Static  | _word32_          .2+| 32-bit relative offset to a function or its PLT entry
+                                            <| S + A - P
+.2+| 60-191  .2+| *Reserved*                          .2+| -       |                   .2+| Reserved for future standard use
                                             <|
 .2+| 192-255 .2+| *Reserved*                          .2+| -       |                   .2+| Reserved for nonstandard ABI extensions
                                             <|