*: 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/setup_environment.c b/libbb/setup_environment.c
index 6e3575c..04e333f 100644
--- a/libbb/setup_environment.c
+++ b/libbb/setup_environment.c
@@ -30,7 +30,7 @@
 
 #include "libbb.h"
 
-void setup_environment(const char *shell, int clear_env, int change_env, const struct passwd *pw)
+void FAST_FUNC setup_environment(const char *shell, int clear_env, int change_env, const struct passwd *pw)
 {
 	if (clear_env) {
 		const char *term;