[MC][AsmParser] Hook for post assembly file processing

This patch handles LLVM standalone assembler (llvm-mc) ELF flag setting based on input file
directive processing.

Mips assembly requires processing inline directives that directly and
indirectly affect the output ELF header flags. This patch handles one
".abicalls".

To process these directives we are following the model the code generator
uses by storing state in a container as we go through processing and when
we detect the end of input file processing, AsmParser is notified and we
update the ELF header flags through a MipsELFStreamer method with a call from
MCTargetAsmParser::emitEndOfAsmFile(MCStreamer &OutStreamer).

This patch will allow other targets the same functionality.

Jack