Silence warnings from clang 10 with -Wimplicit-fallthrough. * lib/dfa.c (FALLTHROUGH): Use __attribute__ __fallthrough__ also on clang >= 10. * lib/fnmatch.c (FALLTHROUGH): Likewise. * lib/fts.c (FALLTHROUGH): Likewise. * tests/macros.h (FALLTHROUGH): Likewise. * lib/regex_internal.h (FALLTHROUGH): Likewise. * config/srclist.txt: Mark it as needing sync with glibc.