*: 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/correct_password.c

@@ -36,7 +36,7 @@
  *
  * NULL pw means "just fake it for login with bad username" */
 
-int correct_password(const struct passwd *pw)
+int FAST_FUNC correct_password(const struct passwd *pw)
 {
 	char *unencrypted, *encrypted;
 	const char *correct;