*: 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/human_readable.c b/libbb/human_readable.c
index d60ef61..dad26ed 100644
--- a/libbb/human_readable.c
+++ b/libbb/human_readable.c
@@ -28,7 +28,7 @@
 
 #include "libbb.h"
 
-const char *make_human_readable_str(unsigned long long size,
+const char* FAST_FUNC make_human_readable_str(unsigned long long size,
 	unsigned long block_size, unsigned long display_unit)
 {
 	/* The code will adjust for additional (appended) units */