[Mips] TargetStreamer Support for .set mips16.

This patch updates .set mips16 support which
affects the ELF ABI and its flags. In addition the patch uses
a common interface for both the MipsTargetSteamer and
MipsObjectStreamer that the assembler uses for
both ELF and ASCII output for these directives.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199851 91177308-0d34-0410-b5e6-96231b3b80d8