[ELF][AArch64] Add R_AARCH64_PLT32 relocation type.

The R_AARCH64_PLT32 relocation type will be documented in the next release
of ELF for the 64-bit Arm Architecture. It is being added in draft state
for the benefit of the position independent vtable feature.

R_AARCH64_PLT32 is very similar to R_AARCH64_PREL32. The intention is to
provide a signed 32-bit integer representing an offset from the place
to a function.
- It relocates 32-bit data
- The expression is S + A - P
- The overflow check for the expression is -2^31 <= X < 2^31
- The relocation generates Thunks/Veneers/Stubs and PLT entries as per
  R_AArch64_CALL26
- If the symbol S is an undefined weak the ABI does not define its value.

The ABI defines a code for ilp32 for completeness, I have added the code
but have only added to the existing reloc-types-elf-aarch64.text as there
is no ilp32 equivalent.

Differential Revision: https://reviews.llvm.org/D77647

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