[llvm-objdump] Support detection of feature bits from the object and implement this for Mips.

Summary:
The Mips implementation only covers the feature bits described by the ELF
e_flags so far. Mips stores additional feature bits such as MSA in the
.MIPS.abiflags section.

Also fixed a small bug this revealed where microMIPS wouldn't add the
EF_MIPS_MICROMIPS flag when using -filetype=obj.

Reviewers: echristo, rafael

Subscribers: rafael, mehdi_amini, dsanders, sdardis, llvm-commits

Differential Revision: http://reviews.llvm.org/D21125

llvm-svn: 272880