[libc] Enable running libc unit tests on AMDGPU

The previous patches added the necessary support for global constructors
used to register tests. This patch enables the AMDGPU 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.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D149517