fix(aarch64): allow build with ARM_ARCH_MINOR=4

fix(aarch64): allow build with ARM_ARCH_MINOR=4

When building the FVP platform with SPMD (which activates the context
switch code), but keeping ARM_ARCH_MINOR to 4 or lower, the assembler
will complain about the SCXTNUM_EL2 system register not being supported
by the "selected processor".

Allow building this combination of options by defining the SCXTNUM_EL2
register via the generic S3_ encoding, so any assembler, with any -march
settings, will generate the access without any warnings.

We do protect accesses to this register by runtime checks, if not
explicitly requested otherwise, so can override the toolchain in this
case.

Change-Id: I0941f4c4dcf541bd968c153b9c3fac61ca23f7ef
Signed-off-by: Andre Przywara <andre.przywara@arm.com>