[mips] Interrupt attribute support for mips32r2+.
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