um: Enable ARCH_HAS_GCOV_PROFILE_ALL

Enable ARCH_HAS_GCOV_PROFILE_ALL so that CONFIG_GCOV_PROFILE_ALL can be
selected on UML.  I didn't need to explicitly disable GCOV on anything
to get this to work on the configs I tested.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Richard Weinberger <richard@nod.at>