[ELF] - Linkerscript: Implemented >> and <<

Found this operators used in the wild scripts, for example:

__got2_entries = (_FIXUP_TABLE_ - _GOT2_TABLE_) >>2;
__fixup_entries = (. - _FIXUP_TABLE_)>>2;

Differential revision: https://reviews.llvm.org/D24860

llvm-svn: 282243