*: 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

diff --git a/libbb/die_if_bad_username.c b/libbb/die_if_bad_username.c
index 337ac60..602aadc 100644
--- a/libbb/die_if_bad_username.c
+++ b/libbb/die_if_bad_username.c
@@ -16,7 +16,7 @@
  * at the end of the username.
  */
 
-void die_if_bad_username(const char *name)
+void FAST_FUNC die_if_bad_username(const char *name)
 {
 	goto skip; /* 1st char being dash isn't valid */
 	do {