[COMMITTED] posix: Fix regex_internal.h on bootstrap

[Adhemerval Zanella]

Only define FALLTHROUGH for _LIBC and do not check __clang_major__
value.

It partially syncs with gnulib 5c52f00c69f39fe.

Checked with build-many-glibcs.py for aarch64-linux-gnu,
x86_64-linux-gnu, and s390x-linux-gnu.
---
 posix/regex_internal.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/posix/regex_internal.h b/posix/regex_internal.h
index e31ac92674..011f733176 100644
--- a/posix/regex_internal.h
+++ b/posix/regex_internal.h
@@ -848,12 +848,14 @@ re_string_elem_size_at (const re_string_t *pstr, Idx idx)
 }
 #endif /* RE_ENABLE_I18N */
 
-#ifndef FALLTHROUGH
-# if (__GNUC__ >= 7) || (__clang_major__ >= 10)
+#ifdef _LIBC
+# if __GNUC__ >= 7
 #  define FALLTHROUGH __attribute__ ((__fallthrough__))
 # else
 #  define FALLTHROUGH ((void) 0)
 # endif
+#else
+# include "attribute.h"
 #endif
 
 #endif /*  _REGEX_INTERNAL_H */
-- 
2.25.1