Make the Mips AsmPrinter independent of global subtarget

initialization. Initialize the subtarget once per function and
migrate EmitStartOfAsmFile to either use calls on the
TargetMachine or get information from the subtarget we'd use
for assembling.

The top-level-ness of the MIPS attribute output for assembly is,
by nature, contrary to how we'd want to do this for an LTO
situation where we have multiple cpu architectures so this
solution is good enough for now.

llvm-svn: 229596