*: 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

archival/libunarchive/header_list.c

@@ -5,7 +5,7 @@
 #include "libbb.h"
 #include "unarchive.h"
 
-void header_list(const file_header_t *file_header)
+void FAST_FUNC header_list(const file_header_t *file_header)
 {
 	puts(file_header->name);
 }