selftests/intel_pstate: Fix build rule for x86

selftests/intel_pstate: Fix build rule for x86

Ensure that ARCH is defined and that this only builds for
x86 architectures.

It is possible to build from the root of the Linux tree, which
will define ARCH, or to run make from the selftests/ directory
itself, which has no provision for defining ARCH, so this
change is to use the current definition (if any), or to check
uname -m if undefined.

Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>