This commit introduce 'USE_SWITCH' feature into cortex-A/R(aarch32)
architecture
For introducing USE_SWITCH, the exception entry and exit are unified via
`z_arm_cortex_ar_enter_exc` and `z_arm_cortex_ar_exit_exc`. All
exceptions including ISR are using this way to enter and exit exception
handler.
Differentiate exception depth and interrupt depth. Allow doing
context switch when exception depth greater than 1 but not allow doing
this when interrupt depth greater than 1.
Currently, USE_SWITCH doesn't support FPU_SHARING and USERSPACE.
Signed-off-by: Huifeng Zhang <Huifeng.Zhang@arm.com>
abde709b5e