[mips] Switch asm to __asm__ for non-GNU compiles.

Summary:
Using asm works fine for gnu11, but fails if the compiler uses C11.
Switch to the more consistent __asm__, since that is what the rest of
the source is using.

Reviewers: petarj

Reviewed By: petarj

Subscribers: llvm-commits, sdardis, arichardson, pirama

Differential Revision: https://reviews.llvm.org/D35756

llvm-svn: 308922