ARM: Refactor interrupt_enable/disable/restore.
Allow manipulation with PSR_A bit on ARMv6+.
Remove declaration of unused functions.

This effectively enables asynchronous aborts on early bootstrap stage,
which previously was not enabled due to an error in enable_interrupts().

PR: 201434
Reported by: Gregory Soutade <soutade at gmail.com>
Approved by: kib (mentor)