Documentation/features: Add kcov

Add an arch support status file for kcov (ARCH_HAS_KCOV).

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Link: https://lore.kernel.org/r/20200627092300.29438-1-tklauser@distanz.ch


Signed-off-by: Jonathan Corbet <corbet@lwn.net>