[lld-macho] Support X86_64_RELOC_SIGNED_{1,2,4}

We currently only support extern relocations.
`X86_64_RELOC_SIGNED_{1,2,4}` are like X86_64_RELOC_SIGNED, but with the
implicit addend fixed to 1, 2, and 4, respectively.
See the comment in `lib/Target/X86/MCTargetDesc/X86MachObjectWriter.cpp RecordX86_64Relocation`.

Reviewed By: int3

Differential Revision: https://reviews.llvm.org/D79311