runtime: fix mkversion to output valid path separators

In a GOROOT path a backslash is a path separator
not an escape character. For example, `C:\go`.
Fixes gotest error:
version.go:3: unknown escape sequence: g

R=rsc
CC=golang-dev
https://golang.org/cl/4437076

src/pkg/runtime/mkversion.c

@@ -4,7 +4,7 @@
 char *template =
 	"// generated by mkversion.c; do not edit.\n"
 	"package runtime\n"
-	"const defaultGoroot = \"%s\"\n"
+	"const defaultGoroot = `%s`\n"
 	"const theVersion = \"%s\"\n";
 
 void