Adds a basic interrupt module which for now only gives you the
ability to determine if you're executing in an interrupt
context.
In addition this also adds the first two backends of this module's
context facade (pw_interrupt_cortex_m:context_{armv7m,armv8m}).
Change-Id: If0056997f814b0ac33cb0209fc9b247c2599594e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/24421
Commit-Queue: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
