[build] Support mips toolchain for cross-compiling

This enables compiling V8 stand-alone for mips (big endian)
on an ia32 host with gn args:

custom_toolchain="//:mips-bundled"
v8_use_snapshot=false
v8_enable_i18n_support=false
icu_use_data_file=false
is_debug=false
target_cpu="mips"

Bug: chromium:632390
Change-Id: I0b59126d742a2979dd0fcbcdd3a574008a09479a
Reviewed-on: https://chromium-review.googlesource.com/904990Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51167}

tools/toolchain/BUILD.gn

+# Copyright 2018 the V8 project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/toolchain/gcc_toolchain.gni")
+
+gcc_toolchain("mips-bundled") {
+  toolprefix = rebase_path("//tools/mips_toolchain/bin/mips-mti-linux-gnu-",
+                           root_build_dir)
+  cc = "${toolprefix}gcc"
+  cxx = "${toolprefix}g++"
+
+  readelf = "${toolprefix}readelf"
+  nm = "${toolprefix}nm"
+  ar = "${toolprefix}ar"
+  ld = cxx
+
+  toolchain_args = {
+    current_cpu = "mips"
+    current_os = "linux"
+    is_clang = false
+  }
+}