ARM: v7m: enable support for IRQ stacks

Enable support for IRQ stacks on !MMU, and add the code to the IRQ entry
path to switch to the IRQ stack if not running from it already.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Marc Zyngier <maz@kernel.org>
Tested-by: Vladimir Murzin <vladimir.murzin@arm.com> # ARMv7M