Build: Only pass -mcmse switch for secure compiles

Don't define the C macro __ARM_FEATURE_CMSE at all, as that is done by
the compiler by passing the -mcmse (or equivalent) switch to the
compiler command. Also don't pass command line switch -mcmse to compiler
in case of NS build.

Change-Id: If9ab8e879b81b63b97c6785320225537004b35b1
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>