[MC][CodeGen] Define R_RISCV_PLT32 and lower dso_local_equivalent to it

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

This is needed to support relative vtables on RISCV.

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

The lld handling of this reloc is D143115.

Differential Revision: https://reviews.llvm.org/D143226