[mips] Emit R_MICROMIPS_HIGHER / R_MICROMIPS_HIGHEST relocations

Emit R_MICROMIPS_HIGHER / R_MICROMIPS_HIGHEST relocations for %higher()
and %highest() expressions in case of microMIPS. These relocations do
exactly the same things as R_MIPS_HIGHER / R_MIPS_HIGHEST, but for
consistency it's better to write microMIPS variants.

Differential Revision: http://reviews.llvm.org/D47219

llvm-svn: 333407