cortex-m0/task: Check if interrupts are enabled before switching task

Switching task with disabled interrupts leads to Hard Fault on
Cortex-M0 because:
- SVCall exception have configurable priority (full list can be found
  at 2.3.2 Exception types PM0215 p.23)
- We are using 'cpsid i' to disable interrupts. This instruction sets
  PRIMASK bit (3.7.2 CPSID CPSIE PM0215 p.62)
- When PRIMASK bit is set, all exceptions with configurable priority
  are disabled (PM0215 p.16), so SVCall is masked too

If Hard Fault is inevitable, it will be a good idea to catch this
earlier. It will save time spent debugging why Forced Hard Fault
happens. In functions responsible for enabling, disabling or making
task ready we postpone task switch when interrupts are disabled

BUG=b:190597666
BRANCH=none
TEST=Compile and flash EC on boards with Cortex-M0 and make
     sure that it works properly.

Signed-off-by: Patryk Duda <pdk@semihalf.com>
Change-Id: Id3be74e977ae5d5eed79aad78ee378fa413ed4ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2953229
Commit-Queue: Marcin Wojtas <mwojtas@google.com>
Tested-by: Patryk Duda <patrykd@google.com>
Reviewed-by: Aseda Aboagye <aaboagye@chromium.org>

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