refactor(cm): make SVE and SME build dependencies logical

refactor(cm): make SVE and SME build dependencies logical

Currently, enabling SME forces SVE off. However, the SME enablement
requires SVE to be enabled, which is reflected in code. This is the
opposite of what the build flags require.

Further, the few platforms that enable SME also explicitly enable SVE.
Their platform.mk runs after the defaults.mk file so this override never
materializes. As a result, the override is only present on the
commandline.

Change it to something sensible where if SME is on then code can rely on
SVE being on too. Do this with a check in the Makefile as it is the more
widely used pattern. This maintains all valid use cases but subtly
changes corner cases no one uses at the moment to require a slightly
different combination of flags.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: If7ca3972ebc3c321e554533d7bc81af49c2472be