[libc] Add `LIBC_GPU_TEST_ARCHITECTURE` option to set architecture

Currently, the plan is to support testing on a single GPU architecture.
We query the supported architectures from the user's system. However,
there are times when the user would want to override this. This patch
adds the `LIBC_GPU_TEST_ARCHITECTURE` option, which allows users to
specify which GPU architecture to build for.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D143400