*: 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/find_root_device.c b/libbb/find_root_device.c
index 9779f7e..ca46bf5 100644
--- a/libbb/find_root_device.c
+++ b/libbb/find_root_device.c
@@ -62,7 +62,7 @@
 	return retpath;
 }
 
-char *find_block_device(const char *path)
+char* FAST_FUNC find_block_device(const char *path)
 {
 	struct arena a;