compiler-gcc: document minimum version for `__no_sanitize_coverage__`

The attribute was added in GCC 12.1.

This will simplify future cleanups, and is closer to what we do
in `compiler_attributes.h`.

Link: https://godbolt.org/z/MGbT76j6G
Link: https://lkml.kernel.org/r/20221021115956.9947-5-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Marco Elver <elver@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Dan Li <ashimida@linux.alibaba.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/compiler-gcc.h

@@ -92,6 +92,9 @@
 
 #define __no_sanitize_undefined __attribute__((__no_sanitize_undefined__))
 
+/*
+ * Only supported since gcc >= 12
+ */
 #if defined(CONFIG_KCOV) && __has_attribute(__no_sanitize_coverage__)
 #define __no_sanitize_coverage __attribute__((__no_sanitize_coverage__))
 #else