[MC] [X86] Teach leaq _GLOBAL_OFFSET_TABLE(%rip), %r15 to use R_X86_64_GOTPC32...

[MC] [X86] Teach leaq _GLOBAL_OFFSET_TABLE(%rip), %r15 to use R_X86_64_GOTPC32 instead of R_X86_64_PC32

Summary:
This is similar to D46319 (ARM). x86-64 psABI p40 gives an example:

  leaq _GLOBAL_OFFSET_TABLE(%rip), %r15 # GOTPC32 reloc

GNU as creates R_X86_64_GOTPC32. However, MC currently emits R_X86_64_PC32.

Reviewers: javed.absar, echristo

Subscribers: kristof.beyls, llvm-commits, peter.smith, grimar

Differential Revision: https://reviews.llvm.org/D47507

llvm-svn: 334515