[mips][ias] Fix R_MICROMIPS_GOT16 evaluation and eliminate symbol for R_MICROMIPS_(GOT|HI|LO)16

Summary:
The failure r269410 worked around turned out to be caused by an incorrect
evaluation of R_MICROMIPS_GOT16 which then caused the GOT entries to be
incorrect.

This patch fixes the evaluation and reverts r269410.

Reviewers: sdardis, vkalintiris, rafael

Subscribers: rafael, dsanders, sdardis, llvm-commits

Differential Revision: http://reviews.llvm.org/D20242

llvm-svn: 269641