*: 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/full_write.c

@@ -14,7 +14,7 @@
  * This does multiple writes as necessary.
  * Returns the amount written, or -1 on an error.
  */
-ssize_t full_write(int fd, const void *buf, size_t len)
+ssize_t FAST_FUNC full_write(int fd, const void *buf, size_t len)
 {
 	ssize_t cc;
 	ssize_t total;