kcov: improve CONFIG_ARCH_HAS_KCOV help text

The help text for CONFIG_ARCH_HAS_KCOV is stale, and describes the
feature as being enabled only for x86_64, when it is now enabled for
several architectures, including arm, arm64, powerpc, and s390.

Let's remove that stale help text, and update it along the lines of hat
for ARCH_HAS_FORTIFY_SOURCE, better describing when an architecture
should select CONFIG_ARCH_HAS_KCOV.

Link: http://lkml.kernel.org/r/20190412102733.5154-1-mark.rutland@arm.com
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>