*: 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/safe_write.c

@@ -9,7 +9,7 @@
 
 #include "libbb.h"
 
-ssize_t safe_write(int fd, const void *buf, size_t count)
+ssize_t FAST_FUNC safe_write(int fd, const void *buf, size_t count)
 {
 	ssize_t n;