refactor(cpufeat): check FEAT_FGT in a new way

refactor(cpufeat): check FEAT_FGT in a new way

To implement proper runtime checking of features, and to be able to
extend feat_detect.c to catch other cases, rework the FEAT_FGT check to
directly call a generic function instead of providing a trivial specific
one. The #ifdef is moved into the function, and rewritten as a proper C
if statement.
We need to force the compiler to inline that function, otherwise the
optimisation won't work, once we exceed a certain number of callers.

This starts with FEAT_FGT, but all the other features will be moved over
eventually, in separate patches.

For all features checked this way, we delay the panic() until after
every feature has been checked, to list them all during one run.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: Ic576922ff2c4f8d3c1b87b5843b3626729fe4514