[JITLink][x86-64] Lift GOT, PLT table managers into x86_64.h; reuse for MachO.

This lifts the global offset table and procedure linkage table builders out of
ELF_x86_64.h and into x86_64.h, renaming them with generic names
x86_64::GOTTableBuilder and x86_64::PLTTableBuilder. MachO_x86_64.cpp is updated
to use these classes instead of the older PerGraphGOTAndStubsBuilder tool.