This reverts commit r200064 and r200051.

r200064 depends on r200051.

r200051 is broken: I tries to replace .mips_hack_elf_flags, which is a good
thing, but what it replaces it with is even worse.

The new emitMipsELFFlags it adds corresponds to no assembly directive, is not
marked as a hack and is not even printed to the .s file.

The patch also introduces more uses of hasRawTextSupport.

The correct way to remove .mips_hack_elf_flags is to have the mips target
streamer handle the default flags (and command line options). That way the
same code path is used for asm and obj. The streamer interface should *really*
correspond to what is printed in the .s file.