[ELF] - Implemented R_*_IRELATIVE relocations for x86, x64 targets.

This relocation is similar to R_*_RELATIVE except that the value used in this relocation is the program address returned by the function, which takes no arguments, at the address of
the result of the corresponding R_*_RELATIVE relocation as specified in the processor-specific ABI. The purpose of this relocation to avoid name lookup for locally defined STT_GNU_IFUNC symbols at load-time.

More info can be found in ifunc.txt from https://sites.google.com/site/x32abi/documents.

Differential revision: http://reviews.llvm.org/D15235

llvm-svn: 256144