refactor(cpufeat): decouple FGT feature detection and build flags

refactor(cpufeat): decouple FGT feature detection and build flags

Split the feature check for FEAT_FGT into two parts:
- A boolean function that just evaluates whether the feature is usable.
  This takes build time flags into account, and only evaluates the CPU
  feature ID registers when the flexible FEAT_STATE_CHECK method is
  used.
- A "raw" function that returns the unfiltered CPU feature ID register.

Change the callers where needed, to give them the version they actually
want.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: I9a041132d280451f5d9f653a62904f603b2a916d