gallium: auxiliary: Fix standalone Android build of u_cpu_detect (v2)

gallium: auxiliary: Fix standalone Android build of u_cpu_detect (v2)

Commit 463b7d0332c5("gallium: Enable ARM NEON CPU detection.")
introduced CPU feature detection based Android cpufeatures library.
Unfortunately it also added an assumption that if PIPE_OS_ANDROID is
defined, the library is also available, which is not true for the
standalone build without using Android build system.

Fix it by defining HAS_ANDROID_CPUFEATURES in Android.mk and replacing
respective #ifdefs to use it instead.

v2:
 - Add a comment explaining why the separate flag is needed (Emil).

Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>