*: introduce and use FAST_FUNC: regparm on i386, otherwise no-on

text    data     bss     dec     hex filename
 808035     611    6868  815514   c719a busybox_old
 804472     611    6868  811951   c63af busybox_unstripped

libbb/isdirectory.c

@@ -15,7 +15,7 @@
  * Return TRUE if fileName is a directory.
  * Nonexistent files return FALSE.
  */
-int is_directory(const char *fileName, const int followLinks, struct stat *statBuf)
+int FAST_FUNC is_directory(const char *fileName, const int followLinks, struct stat *statBuf)
 {
 	int status;
 	struct stat astatBuf;