*: 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/xgetcwd.c

@@ -14,7 +14,7 @@
    If argument is not NULL (previous usage allocate memory), call free()
 */
 
-char *
+char* FAST_FUNC
 xrealloc_getcwd_or_warn(char *cwd)
 {
 #define PATH_INCR 64