arch: arm: conditionally select FP extension in cortex-m MCUs

This commit conditionally selects the ARMV7_M_ARMV8_M_FP option
in ARMv7-M/ARMv8-M Mainline processors, when the Floating Point
Extension is implemented (CPU_HAS_FPU is selected).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>