[libc] Silence integer shortening warnings on NVPTX masks

Nvidia uses a 32-bit mask, but we store it in a common 64-bit integer to
provide it with a compatible ABI with the AMD implementaiton which may
use a 64-bit mask. Silence these warnings by explicitly casting to the
smaller value, we know this is always legal as the result will always
fit into the smaller value if it was generated on NVPTX.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D157548