Support for 64-bit PC-relative relocations for X86_64

ELF files generated for X86_64 targets may contain 64-bit PC-relative 
relocations. For instance, an exception handler table entry contains the start 
of exception-throwing frame relative to the start of exception handler. As these 
two labels belong to different sections, their difference and so the relocation 
is 64-bit.

An attempt to parse such file, i.e. in DWARFContext::create, results in "failed 
to compute relocation" error.

This fix adds support for such relocations to RelocationResolver.cpp.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D67779

Patch by Oleg Pliss (Oleg.Pliss@azul.com)

llvm-svn: 372447