Optimize ffs() implementation

- Use gcc builtin
- Define as a static inline function in the .h so no function call overhead is needed
- Keep the function in libroot for backwards compatibility
- Remove a duplicate implementation in the freebsd compatibility layer

gcc2 does not document the builtin, but it is in fact already available
there as well.

Fixes #3281.

Change-Id: I94f8a2548637aa70e85febbfab06f07c1a427005
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2605
Reviewed-by: waddlesplash <[email protected]>

headers/posix/strings.h

@@ -13,7 +13,7 @@
 extern "C" {
 #endif
 
-extern int ffs(int i);
+static inline int ffs(int i) { return __builtin_ffs(i); }
 
 extern int		strcasecmp(const char *string1, const char *string2);
 extern int		strncasecmp(const char *string1, const char *string2,