Fix __has_attribute(__no_sanitize_coverage__) for GCC 4 by defining
__GCC4_has_attribute___no_sanitize_coverage__.

Fixes: 540540d0 ("kcov: add __no_sanitize_coverage to fix noinstr for all architectures")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>