*: 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/update_passwd.c

@@ -36,7 +36,7 @@ static void check_selinux_update_passwd(const char *username)
 #define check_selinux_update_passwd(username) ((void)0)
 #endif
 
-int update_passwd(const char *filename, const char *username,
+int FAST_FUNC update_passwd(const char *filename, const char *username,
 			const char *new_pw)
 {
 	struct stat sb;