[ELF] Provide the GNU hash function in libObject

GNU uses a different hashing function compared to the sys-V standard
function already provided in libObject. This is already used internally
in LLD for generating synthetic sections. This patch simply extracts
this definition and makes it availible to other users of `libObject`.
This is done in preparation for supporting symbol name lookups via the
GNU hash table.

Reviewed By: MaskRay, jhenderson

Differential Revision: https://reviews.llvm.org/D132696