*: 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/wfopen.c

@@ -9,7 +9,7 @@
 
 #include "libbb.h"
 
-FILE *fopen_or_warn(const char *path, const char *mode)
+FILE* FAST_FUNC fopen_or_warn(const char *path, const char *mode)
 {
 	FILE *fp = fopen(path, mode);
 	if (!fp) {