[aaelf64] Clarify addend for R_AARCH64_GOTPCREL32

[smithp35]

Define the expression for R_AARCH64_GOTPCREL32 as GDAT(S)-P+A. This matches the only implementation in clang and lld.

The relocation is used to calculate the offset from the start of the vtable to a GOT entry that contains the address of the RTTI object. As the table entry for the RTTI pointer is at an offset from the start of the vtable the relocation addend contains -offset to cancel out.

Previously in https://github.com/ARM-software/abi-aa/pull/272 the relocation definition of relocations using GDAT(S+A) were changed to require A to be 0 as lld and GNU ld were implementing GDAT(S+A) as GDAT(S) + A and GDAT(S) + 0 respectively.

As this specific relocation is only implemented in clang and lld it is safe to update the description to match the implementation without affecting portability.

We use GDAT(S)-P+A rather than GDAT(S) + A - P as the latter implies that we are calculating an offset to a different GOT slot to GDAT(S) rather than an offset from P.

Discussion and example: https://github.com/ARM-software/abi-aa/pull/272