configure.ac: fix host_cpu/atomics detection

Previous code was busted, as it wasn't checking directly for what it was
meant to, and at the end changing the user's selection if host_cpu
heuristics were involved.

Simplify things by adding a macro that does the long message printing
for us, and check for only what we need.

This fixes commit 36cff14b(configure: omap, freedreno and tegra
require atomics) which incorrectly assumed that the code was working
fine, and effectively made impossible to enable freedreno due to it's
host_cpu detection.

Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>