enable mutex lowering and hardware floating point on gnueabihf. closes #10482
auto merge of #12852 : itdaniher/rust/master, r=alexcrichton

This enables the lowering of llvm 64b intrinsics to hardware ops, resolving issues around `__kernel_cmpxchg64` on older kernels on ARM devices, and also enables use of the hardware floating point unit, resolving https://github.com/mozilla/rust/issues/10482.