fix MIPS target

I ignored AtomicU64 methods on MIPS target
because libgcc doesn't implement MIPS32 64-bit atomic operations.
Otherwise it would cause link failure.
auto merge of #12878 : crabtw/rust/mips, r=alexcrichton

I ignored AtomicU64 methods on MIPS target
because libgcc doesn't implement MIPS32 64-bit atomic operations.
Otherwise it would cause link failure.

By the way, the patched LLVM doesn't have MIPS split stack anymore.
Should I file an issue about that?