cmake: Improve the error message when the compiler is not executable

When the SDK/toolchain has been incorrectly installed such that the
user does not have permissions to execute it an incomprehensible
errror message occurs.

This patch tests that executing 'gcc --version' does not give an error
code and errors out with an appropriate error message if it does.

This fixes #8977

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