runtime: Correct syscall.Setenv for systems that don't have setenv.

From-SVN: r186911

libgo/runtime/go-setenv.c

@@ -50,7 +50,7 @@ setenv_c (struct __go_string k, struct __go_string v)
 
 #else /* !defined(HAVE_SETENV) */
 
-  kn = malloc (k.__length + v.__length + 2);
+  kn = __go_alloc (k.__length + v.__length + 2);
   __builtin_memcpy (kn, ks, k.__length);
   kn[k.__length] = '=';
   __builtin_memcpy (kn + k.__length + 1, vs, v.__length);