*: 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/getopt32.c

@@ -316,7 +316,7 @@ const char *applet_long_options;
 
 uint32_t option_mask32;
 
-uint32_t
+uint32_t FAST_FUNC
 getopt32(char **argv, const char *applet_opts, ...)
 {
 	int argc;