*: 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/mode_string.c b/libbb/mode_string.c
index d17cc4a..b9975f4 100644
--- a/libbb/mode_string.c
+++ b/libbb/mode_string.c
@@ -51,7 +51,7 @@
 /*                                  0123456789abcdef */
 static const char mode_chars[7] ALIGN1 = "rwxSTst";
 
-const char *bb_mode_string(mode_t mode)
+const char* FAST_FUNC bb_mode_string(mode_t mode)
 {
 	static char buf[12];
 	char *p = buf;
@@ -91,7 +91,7 @@
 /*                                  0123456789abcdef */
 static const char mode_chars[7] = "rwxSTst";
 
-const char *bb_mode_string(mode_t mode)
+const char* FAST_FUNC bb_mode_string(mode_t mode)
 {
 	static char buf[12];
 	char *p = buf;