[ELF] - Added support for jmp/call relaxations when...

[ELF] - Added support for jmp/call relaxations when R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX are used.

D15779 introduced basic approach to support new relaxations.
This patch implements relaxations for jmp and call instructions,
described in System V Application Binary Interface AMD64 Architecture Processor 
Supplement Draft Version 0.99.8 (https://github.com/hjl-tools/x86-psABI/wiki/x86-64-psABI-r249.pdf, 
B.2 "B.2 Optimize GOTPCRELX Relocations")

Differential revision: http://reviews.llvm.org/D20622

llvm-svn: 270721