arch: arm: aarch32: Fix arch_cpu_idle interrupt masking

The current AArch32 `arch_cpu_idle` implementation enables interrupt
before executing the WFI instruction, and this has the side effect of
allowing interruption and thereby calling wake-up notification
functions before the CPU enters sleep.

This commit fixes the problem described above by ensuring that
interrupt is disabled when the WFI instruction is executed and
re-enabled only after the processor wakes up.

For ARMv6-M, ARMv8-M Baseline and ARM-R, the PRIMASK (ARM-M)/
CPSR.I (ARM-R) is used to lock interrupts and therefore it is not
necessary to do anything before executing the WFI instruction.

For ARMv7-M and ARMv8-M Mainline, the BASEPRI is used to lock
interrupts and the PRIMASK is always cleared in non-interrupt context;
therefore, it is necessary to set the PRIMASK to mask interrupts,
before clearing the BASEPRI to configure wake-up interrupt priority to
the lowest.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>