*: 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/copy_file.c

@@ -71,7 +71,7 @@ static int ask_and_unlink(const char *dest, int flags)
  *  0 copy is made or user answered "no" in interactive mode
  *    (failures to preserve mode/owner/times are not reported in exit code)
  */
-int copy_file(const char *source, const char *dest, int flags)
+int FAST_FUNC copy_file(const char *source, const char *dest, int flags)
 {
 	/* This is a recursive function, try to minimize stack usage */
 	/* NB: each struct stat is ~100 bytes */