[mips] Implement GetWriteFlag() for mips

The read/write flag is set by manually decoding the instruction that caused
the exception. It is implemented this way because the cause register which
contains the needed flag was removed from the signal context structure which
the user handler receives from the kernel.

Patch by Milos Stojanovic.

Differential Revision: https://reviews.llvm.org/D45768

llvm-svn: 330840