[libc] Default to a single threaded thread pool for GPU tests (#74486)

Summary:
The GPU tests tend to fail when run massively in parallel. This is why
we use a CMake job pool to limit it to 1 in most cases. We should
default to the configuration that is most likely to work, that being a
single thread. There aren't enough GPU tests for this to be a massive
increase in test time on the bots, so we should default to what works
guaranteed.

Author: jhuber6

libc/cmake/modules/prepare_libc_gpu_build.cmake

@@ -78,6 +78,9 @@ set(LIBC_GPU_TEST_JOBS "" CACHE STRING "Number of jobs to use for GPU tests")
 if(LIBC_GPU_TEST_JOBS)
   set_property(GLOBAL PROPERTY JOB_POOLS LIBC_GPU_TEST_POOL=${LIBC_GPU_TEST_JOBS})
   set(LIBC_HERMETIC_TEST_JOB_POOL JOB_POOL LIBC_GPU_TEST_POOL)
+else()
+  set_property(GLOBAL PROPERTY JOB_POOLS LIBC_GPU_TEST_POOL=1)
+  set(LIBC_HERMETIC_TEST_JOB_POOL JOB_POOL LIBC_GPU_TEST_POOL)
 endif()
 
 set(LIBC_GPU_TEST_ARCHITECTURE "" CACHE STRING "Architecture for the GPU tests")