*: 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/crypt_make_salt.c b/libbb/crypt_make_salt.c
index ebdf024..393eba5 100644
--- a/libbb/crypt_make_salt.c
+++ b/libbb/crypt_make_salt.c
@@ -24,7 +24,7 @@
 	return ('a' - 38 + i);
 }
 
-int crypt_make_salt(char *p, int cnt, int x)
+int FAST_FUNC crypt_make_salt(char *p, int cnt, int x)
 {
 	x += getpid() + time(NULL);
 	do {