Add register Mips::GP to the list of reserved registers if target is bare-metal

Add register Mips::GP to the list of reserved registers if target is bare-metal
to prevent it from being clobbered. mips uses $gp to access small data section.

This bug was originally reported by Carl Norum.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162340 91177308-0d34-0410-b5e6-96231b3b80d8