*: 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/strrstr.c

@@ -19,7 +19,7 @@
  * The strrstr() function finds the last occurrence of the substring needle
  * in the string haystack. The terminating nul characters are not compared.
  */
-char* strrstr(const char *haystack, const char *needle)
+char* FAST_FUNC strrstr(const char *haystack, const char *needle)
 {
 	char *r = NULL;