selftests/intel_pstate: fix build for ARCH=x86_64

[ Upstream commit beb7d862ed4ac6aa14625418970f22a7d55b8615 ]

Handle the scenario where the build is launched with the ARCH envvar
defined as x86_64.

Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>