refactor(context-mgmt): move FEAT_FGT save/restore code into C

refactor(context-mgmt): move FEAT_FGT save/restore code into C

At the moment we do the EL2 context save/restore sequence in assembly,
where it is just guarded by #ifdef statement for the build time flags.
This does not cover the FEAT_STATE_CHECK case, where we need to check
for the runtime availability of a feature.

To simplify this extension, and to avoid writing too much code in
assembly, move that sequence into C: it is called from C context
anyways.

This protects the C code with the new version of the is_xxx_present()
check, which combines both build time and runtime check, as necessary,
and allows the compiler to optimise the calls aways, if we don't need
them.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: I7c91bec60efcc00a43429dc0381f7e1c203be780