*: 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/error_msg_and_die.c b/libbb/error_msg_and_die.c
index addd818..243433b 100644
--- a/libbb/error_msg_and_die.c
+++ b/libbb/error_msg_and_die.c
@@ -9,7 +9,7 @@
 
 #include "libbb.h"
 
-void bb_error_msg_and_die(const char *s, ...)
+void FAST_FUNC bb_error_msg_and_die(const char *s, ...)
 {
 	va_list p;