[libc] Allow NVPTX to use aliases

Summrary:
Following D156014 we can now use aliases for NVPTX, removing this source
of divergence. We require at least +ptx63 and at least sm_30 for
`.alias` but this is already within what we build for with `libc`
support.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D157323