*: 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/kernel_version.c

@@ -16,7 +16,7 @@
  *
  *     if (get_linux_version_code() > KERNEL_VERSION(2,2,11)) { <stuff> }
  */
-int get_linux_version_code(void)
+int FAST_FUNC get_linux_version_code(void)
 {
 	struct utsname name;
 	char *s;