[lld-macho] Support X86_64_RELOC_GOT

Summary:
As far as I can tell, it's identical to _GOT_LOAD. llvm-mc has the following
comment explaining why _GOT exists:

```
// x86_64 distinguishes movq foo@GOTPCREL so that the linker can
// rewrite the movq to an leaq at link time if the symbol ends up in
// the same linkage unit.
```

Depends on D80855.

Reviewers: ruiu, pcc, MaskRay, smeenai, alexshap, gkm, Ktwu, christylee

Reviewed By: MaskRay, smeenai

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80856