[mips] Add initial (experimental) MIPS-IV support.

Summary:
Adds the 'mips4' processor and a simple test of the ELF e_flags.

Patch by David Chisnall
His work was sponsored by: DARPA, AFRL

I made one small change to the testcase so that it uses
mips64-unknown-linux instead of mips4-unknown-linux.

This patch indirectly adds FeatureCondMov to FeatureMips64. This is ok
because it's supposed to be there anyway and it turns out that
FeatureCondMov is not a predicate of any instructions at the moment
(this is a bug that hasn't been noticed because there are no targets
without the conditional move instructions yet).

CC: theraven

Differential Revision: http://llvm-reviews.chandlerc.com/D3244