[arm] Explicitly set .fpu in cmse_nonsecure_call.S

This file is missing a .fpu directive and was relying on the compiler
driver passing through a -mfpu= command line option.  When the FPU is
auto, that will not be passed through correctly, so set something
suitable within the file itself.

libgcc:
	 * config/arm/cmse_nonsecure_call.S: Explicitly set the FPU.

From-SVN: r249297