*: 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_gethostname.c

@@ -33,7 +33,7 @@
  * This is an illegal first character for a hostname.
  * The returned malloced string must be freed by the caller.
  */
-char *safe_gethostname(void)
+char* FAST_FUNC safe_gethostname(void)
 {
 	struct utsname uts;