[mips] do not use FastISel when -mxgot is present

The clang compiler by default uses FastISel when invoked with -O0, which
is also the default. In that case, passing of -mxgot does not get honored,
i.e. the code path that is to deal with large got is not taken.
Clang produces same output regardless of -mxgot being present or not.
This change checks whether -mxgot is passed as an option, and turns off
FastISel if it is.

Patch by Stefan Maksimovic.

Differential Revision: https://reviews.llvm.org/D33593

llvm-svn: 304906