[mips] Interrupt attribute support for mips32r2+.

Summary:
This patch adds support for using the "interrupt" attribute on Mips
for interrupt handling functions. At this time only mips32r2+ with the
o32 ABI with the static relocation model is supported. Unsupported
configurations will be rejected

Patch by Simon Dardis (+ clang-format & some trivial changes to follow the
LLVM coding standards by me).

Reviewers: mpf, dsanders

Subscribers: dsanders, vkalintiris, llvm-commits

Differential Revision: http://reviews.llvm.org/D10768

llvm-svn: 251286