[libc][AMDGPU] Enable parallel tests for libc

It appears that llvm/llvm-project#70695 fixed an
issue that prevented libc unit tests to be run in parallel.

Author: jplehr

buildbot/osuosl/master/config/builders.py

@@ -1812,7 +1812,6 @@
                             "-DLIBC_GPU_BUILD=ON",
                             "-DLIBC_GPU_ARCHITECTURES=gfx906",
                             "-DLIBC_GPU_TEST_ARCHITECTURE=gfx906",
-                            "-DLIBC_GPU_TEST_JOBS=1",
                             ],
                         install=True,
                         testsuite=False,