runtime: separate out auto-generated files, take 2

This is like the ill-fated CL 5493063 except that
I have written a shell script (autogen.sh) instead of
thinking I could possibly write a correct Makefile.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5496075
diff --git a/src/pkg/runtime/sys_linux_arm.s b/src/pkg/runtime/sys_linux_arm.s
index d0b36de..e0e4135 100644
--- a/src/pkg/runtime/sys_linux_arm.s
+++ b/src/pkg/runtime/sys_linux_arm.s
@@ -6,7 +6,7 @@
 // System calls and other sys.stuff for arm, Linux
 //
 
-#include "asm_arm.h"
+#include "zasm_GOOS_GOARCH.h"
 
 // OABI
 //#define SYS_BASE 0x00900000