[mips][mc] Add basic support for R_MIPS_JALR/R_MICROMIPS_JALR

R_MIPS_JALR/R_MICROMIPS_JALR can now be parsed in .s files and emitted to .o.
They are still not generated with JALR.

Differential revision: https://reviews.llvm.org/D54721

llvm-svn: 347398