[Mips] Support for MicroMips STO refactoring.

No true functional changes.

Change the "hack" name of emitMipsHackSTOCG to emitSymSTO.

Remove demonstration code in AsmParser for emitMipsHackSTOCG and
emitMipsHackELFFlags. The STO field is in an ELF symbol and is not
an explicit directive. That said, we are missing the compliment call
in AsmParser and that will need to be addressed soon.

XFAIL dummy tests for emitMipsHackELFFlags and emitMipsHackELFFlags.
These will built out with following patches.

llvm-svn: 195067