*: 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/archival/libunarchive/get_header_cpio.c b/archival/libunarchive/get_header_cpio.c
index b97b53b..4ed18c6 100644
--- a/archival/libunarchive/get_header_cpio.c
+++ b/archival/libunarchive/get_header_cpio.c
@@ -17,7 +17,7 @@
 	char name[1];
 } hardlinks_t;
 
-char get_header_cpio(archive_handle_t *archive_handle)
+char FAST_FUNC get_header_cpio(archive_handle_t *archive_handle)
 {
 	static hardlinks_t *saved_hardlinks = NULL;
 	static hardlinks_t *saved_hardlinks_created = NULL;