toolchain: add missing patch to gcc v4.8.0

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36511 3c298f89-4303-0410-b956-a3cf2f4a3e73

toolchain/gcc/patches/4.8.0/920-specs_nonfatal_getenv.patch

@@ -0,0 +1,14 @@
+--- a/gcc/gcc.c
++++ b/gcc/gcc.c
+@@ -8003,7 +8003,10 @@ getenv_spec_function (int argc, const ch
+ 
+   value = getenv (argv[0]);
+   if (!value)
+-    fatal_error ("environment variable %qs not defined", argv[0]);
++    {
++      warning (0, "environment variable %qs not defined", argv[0]);
++      value = "";
++    }
+ 
+   /* We have to escape every character of the environment variable so
+      they are not interpreted as active spec characters.  A