util: Add accessor for util_cpu_caps

util: Add accessor for util_cpu_caps

In release builds, there should be no change, but in debug builds the
assert will help us catch undefined behavior resulting from using
util_cpu_caps before it is initialized.

With fix for u_half_test for MSVC from Jesse Natalie squashed in.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9266>