*: 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/last_char_is.c

@@ -12,7 +12,7 @@
 /* Find out if the last character of a string matches the one given.
  * Don't underrun the buffer if the string length is 0.
  */
-char* last_char_is(const char *s, int c)
+char* FAST_FUNC last_char_is(const char *s, int c)
 {
 	if (s && *s) {
 		size_t sz = strlen(s) - 1;