runtime: fix asm.h on Windows

Thanks to mhantsch@gmail.com.

R=adg, brainman
CC=golang-dev
https://golang.org/cl/4092048

src/pkg/runtime/mkasmh.sh

@@ -84,6 +84,7 @@ esac
 echo
 
 awk '
+{ gsub(/\r/, ""); }
 /^aggr G$/ { aggr="g" }
 /^aggr M$/ { aggr = "m" }
 /^aggr Gobuf$/ { aggr = "gobuf" }