rts/sm/Storage.c: tweak __clear_cache proto for clang

clang defines '__clear_cache' slightly differently from gcc:
    rts/sm/Storage.c:1349:13: error:
         error: conflicting types for '__clear_cache'
         |
    1349 | extern void __clear_cache(char * begin, char * end);
         |             ^
    extern void __clear_cache(char * begin, char * end);
                ^
         note: '__clear_cache' is a builtin with type 'void (void *, void *)'

Reported by Moritz Angermann.

While at it used '__builtin___clear_cache' if advertised by clang.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>