[Mips] TargetStreamer ELF flag Support for default and commandline options.

This patch uses a common MipsTargetSteamer interface for both
MipsAsmPrinter and MipsAsmParser for recording default and commandline
driven directives that affect ELF header flags.

It has been noted that the .ll tests affected by this patch belong in
test/Codegen/Mips. I will move them in a separate patch.

Also, a number of directives do not get expressed by AsmPrinter in the
resultant .s assembly such as setting the correct ASI. I have noted this
in the tests and they will be addressed in later patches.