*: 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/pidfile.c b/libbb/pidfile.c
index cafa789..7b8fee2 100644
--- a/libbb/pidfile.c
+++ b/libbb/pidfile.c
@@ -13,7 +13,7 @@
 
 smallint wrote_pidfile;
 
-void write_pidfile(const char *path)
+void FAST_FUNC write_pidfile(const char *path)
 {
 	int pid_fd;
 	char *end;