[mips] Implement the ".rdata" MIPS assembly directive.

Rather than creating a separate ".rdata" section distinct from the
customary ".rodata" in ELF, ".rdata" switches to the ".rodata" section.

This patch relands r305949 and r305950 with the correct commit message
and addresses nit raised during review.

Patch By: John Baldwin!

Differential Revision: https://reviews.llvm.org/D34452

llvm-svn: 305995