*: 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/create_icmp6_socket.c b/libbb/create_icmp6_socket.c
index a22ac5d..2065517 100644
--- a/libbb/create_icmp6_socket.c
+++ b/libbb/create_icmp6_socket.c
@@ -9,7 +9,7 @@
 #include "libbb.h"
 
 #if ENABLE_FEATURE_IPV6
-int create_icmp6_socket(void)
+int FAST_FUNC create_icmp6_socket(void)
 {
 	int sock;
 #if 0