Reland "[mips] Teach the driver to accept -m(no-)gpopt."

This patch teaches the driver to pass -mgpopt by default to the backend when it
is supported, i.e. we are using -mno-abicalls.

Reviewers: atanasyan, slthakur

Differential Revision: https://reviews.llvm.org/D35548

This version fixes a logic error that generated warnings incorrectly and
gets rid of spurious arguments to the backend when -mgpopt is not used.

llvm-svn: 308619