*: 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

diff --git a/libbb/printable.c b/libbb/printable.c
index 676758a..ae93359 100644
--- a/libbb/printable.c
+++ b/libbb/printable.c
@@ -9,7 +9,7 @@
 
 #include "libbb.h"
 
-void fputc_printable(int ch, FILE *file)
+void FAST_FUNC fputc_printable(int ch, FILE *file)
 {
 	if ((ch & (0x80 + PRINTABLE_META)) == (0x80 + PRINTABLE_META)) {
 		fputs("M-", file);