libopcodes/mips: add support for disassembler styling

This commit adds disassembler styling support for MIPS.  After this
commit objdump and GDB will style disassembler output.

This is a pretty straight forward change, we switch to use the
disassemble_info::fprintf_styled_func callback, and pass an
appropriate style through as needed.  No additional tricks were
needed (compared to say i386, or ARM).

Tested by running all of the objdump commands used by the gas
testsuite and manually inspecting the styled output, everything looks
reasonable, though I'm not a MIPS expert, so it is possible that I've
missed some corner cases.  Worst case though is that something will be
styled incorrectly, the actual content should be unchanged.

All the gas, ld, and binutils tests still pass for me.