Cherry-pick r244675: [MIPS] Use arch values for lock-free atomic ops

Original commit message:

[MIPS] Use arch values for lock-free atomic operations

Let NaClMips32ELTargetInfo inherit arch values for maximum width lock-free
atomic operations.

Differential Revision: http://reviews.llvm.org/D11949

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4205
R=dschuff@chromium.org

Review URL: https://codereview.chromium.org/1275423007 .