*: 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

libbb/safe_strncpy.c

@@ -10,7 +10,7 @@
 #include "libbb.h"
 
 /* Like strncpy but make sure the resulting string is always 0 terminated. */
-char *safe_strncpy(char *dst, const char *src, size_t size)
+char* FAST_FUNC safe_strncpy(char *dst, const char *src, size_t size)
 {
 	if (!size) return dst;
 	dst[--size] = '\0';