cmake: Cache the 'toolchain-is-ok' test as well

All compiler flag checks are cached for build-time performance reasons
except for the test that tests the toolchain itself.

It is believed that this test was left uncached because at the time,
there was not enough trust in the caching mechanism. But time has
shown that the caching mechanism is safe. So cache this test as well.

This improves build-time performance and also reduces noise in the
logs.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>