Implement the missing bits corresponding to .mips_hack_elf_flags.

These were:
* noreorder handling on the target object streamer and asm parser.
* setting the initial flag bits based on the enabled features.
* setting the elf header flag for micromips

It is *really* depressing I am the one doing this instead of someone at
mips actually taking the time to understand the infrastructure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200138 91177308-0d34-0410-b5e6-96231b3b80d8