toolchain: clang: arm: do not use fp instruction when CONFIG_FPU=n

Clang uses floating-point instructions by default, even if -mfpu is not
defined. Disable using FPU when CONFIG_FPU=n.

Using floating-point instructions when FPU is not enabled generates
Usage Fault.

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>