arch/Config.in.arm: add FPU related options for ARMv8 cores

arch/Config.in.arm: add FPU related options for ARMv8 cores

The ARMv8 cores have a mandatory FPU unit called FP-ARMv8, so we:

 - add a new hidden Config.in option for the availability of this
   unit (BR2_ARM_CPU_HAS_FP_ARMV8)

 - allow the selection of a possible choice in the "Floating point
   strategy", and add two new choices: BR2_ARM_FPU_FP_ARMV8 and
   BR2_ARM_FPU_NEON_FP_ARMV8.

 - specify the -mfpu values for BR2_ARM_FPU_FP_ARMV8 and
   BR2_ARM_FPU_NEON_FP_ARMV8 cases, when used on ARM 32 bits (-mfpu
   doesn't exist on ARM64, instead -mcpu modifiers are used, so they
   will be added on a per-core basis).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[yann.morin.1998@free.fr: drop the FP strategy dependency]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>