[libc] Enable running libc unit tests on NVPTX

The previous patches added the necessary support for global constructors
used to register tests. This patch enables the NVPTX target to build
and run the unit tests on the GPU. Currently this only tests the ctype
tests, but adding more should be straightforward from here on.

This ran all the ctest unit tests when run on an sm_70.

Depends on D149517 D149527

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D149532