[libc] Add an option to use a job pool for GPU tests

Currently the GPU has restrictions on how many tests can be run in
parallel due to resource constraints. However, building these tests can
take a long time so we want to be able to build them in parallel. This
patch introduces the option `LIBC_GPU_TEST_JOBS` which is set to the
number of threads to run in parallel.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D153157