[cuda] treat file scope __asm as __host__ and ignore it during device-side compilation.

Currently clang emits file-scope asm during *both* host and device
compilation modes which is usually a wrong thing to do.

There's no way to attach any attribute to an __asm statement, so
there's no way to differentiate between host-side and device-side
file-scope asm.  This patch makes clang to match nvcc behavior and
emit file-scope-asm only during host-side compilation.

Differential Revision: http://reviews.llvm.org/D9270

llvm-svn: 235905