cmake: armclang: Handle -mfpu=auto used by cortex-m55

arm-clang utilizes the gcc fpu flags, however -mfpu=auto is not a
supported option in arm-clang.  For arm-clang we can mimic the auto
behavior by just not setting the -mfpu option.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>