syscall: let mkall.sh work on multiarch systems (like Precise)

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6912063

src/pkg/syscall/mkall.sh

@@ -145,8 +145,13 @@ linux_386)
 	mktypes="GOARCH=$GOARCH go tool cgo -godefs"
 	;;
 linux_amd64)
+	unistd_h=$(ls -1 /usr/include/asm/unistd_64.h /usr/include/x86_64-linux-gnu/asm/unistd_64.h 2>/dev/null | head -1)
+	if [ "$unistd_h" = "" ]; then
+		echo >&2 cannot find unistd_64.h
+		exit 1
+	fi
 	mkerrors="$mkerrors -m64"
-	mksysnum="./mksysnum_linux.pl /usr/include/asm/unistd_64.h"
+	mksysnum="./mksysnum_linux.pl $unistd_h"
 	mktypes="GOARCH=$GOARCH go tool cgo -godefs"
 	;;
 linux_arm)