This patch reworks the YAML files for the llext samples and tests to
share a common restriction list. Also, using an arch-specific config
to disable the MPU for the ARM architecture only, there is no need
to duplicate the test cases per architecture.
Use this to enable the "writable" test case for the ARM architecture
and separate the "modules_enabled" case to test building as a module.
Signed-off-by: Luca Burelli <l.burelli@arduino.cc>