arch: arm: re-organize thread stack macro defines in arch.h

This commit re-organizes the macro definitions in arch.h for
the ARM architecture. In particular, the commit:
- defines the minimum alignment requirement for thread stacks,
  that is, excluding alignment requirement for (possible)
  MPU stack guards.
- defines convenience macros for the MPU stack guard align and
  size for threads using the FP services under Shared registers
  mode (CONFIG_FP_SHARING=y). For that, a hidden Kconfig option
  is defined in arch/arm/core/cortex_m/mpu/Kconfig.
- enforces stack alignment with a wide MPU stack guard (128
  bytes) under CONFIG_FP_SHARING=y for the ARMv7-M architecture,
  which requires start address alignment with power-of-two and
  region size.

The commit does not change the amount of stack that is reserved
with K_THREAD_STACK_DEFINE; it only determines the stack buffer
alignment as explained above.

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