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

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

At the moment we save and restore the HCRX_EL2 register in assembly, and
just depend on the build time flags.
To allow runtime checking, and to avoid too much code in assembly, move
that over to C, and use the new combined build/runtime feature check.

This also allows to drop the assert, since this should now be covered by
the different FEAT_STATE_x options.

Change-Id: I3e20b9ba17121d423cd08edc20bbf4e7ae7c0178
Signed-off-by: Andre Przywara <andre.przywara@arm.com>