Fix ffs on x86 for old API levels.

<strings.h>'s ffs used to work by accident. In the past, <strings.h> used
to incorrectly declare ffs for all platforms at all API levels. In the
unified headers, there's no such declaration for x86 before API 18 (which
makes sense, because that function was missing on x86 until then).

But as long as there was a declaration for ffs, the compiler just inlined
__builtin_ffs. There was no problem at link time because the compiler didn't
actually add a reference to the missing ffs symbol.

Restore the old behavior by manually instructing the compiler to inline its
builtin in these cases.

Bug: android/ndk#439
Test: built new NDK 'ffs' test
Change-Id: I840e99f237c86f7cb028a0f67aaa8c6ff3eda245

libc/include/android/legacy_strings_inlines.h

@@ -0,0 +1,44 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#ifndef _ANDROID_LEGACY_STRINGS_INLINES_H_
+#define _ANDROID_LEGACY_STRINGS_INLINES_H_
+
+#include <strings.h>
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+#if defined(__i386__) && __ANDROID_API__ < __ANDROID_API_J_MR2__
+/* Everyone except x86 had ffs since the beginning. */
+static __inline int ffs(int __n) { return __builtin_ffs(__n); }
+#endif
+
+__END_DECLS
+
+#endif

libc/include/strings.h

@@ -54,8 +54,12 @@ __BEGIN_DECLS
 #define bzero(b, len) (void)(__builtin_memset((b), '\0', (len)))
 #endif
 
+#if !defined(__i386__) || __ANDROID_API__ >= __ANDROID_API_J_MR2__
 int ffs(int) __INTRODUCED_IN_X86(18);
+#endif
 
 __END_DECLS
 
+#include <android/legacy_strings_inlines.h>
+
 #endif /* !defined(_STRINGS_H_) */