[mips] Implemented the .hword directive.

Summary:
In order to pass the tests, this required marking R_MIPS_16 relocations
as needing to point to the symbol and not the section.

Reviewers: vkalintiris, dsanders

Subscribers: dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D17200

llvm-svn: 260896