[lld][RISCV] Introduce handling for R_RISCV_PLT32 relocation

This introduces R_RISCV_PLT32, a PC-relative data relocation that takes
the 32-bit relative offset to a function or its PLT entry.

This is needed to support relative vtables on RISCV.

Github PR: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/363

D143226 has the llvm parts.

Differential Revision: https://reviews.llvm.org/D143115