Fix PNaCl toolchain build on Mac

Fix PNaCl toolchain build on Mac

In the new Mac bot setup, Chromium Clang does not find the Mac SDK
automagically and needs an -isysroot switch.  This is consistent with
how Chromium and NaCl builds work on Mac now.

The binutils_x86 build was implicitly using the system compiler and
expecting that to work.  It no longer works on the new Mac bot setup.
This is changed to build with Chromium Clang like all the other
components (on Linux too, for consistency).  In the old 2.24 binutils
base, gold is not compatible with a modern C++ runtime.  Rather than
hack up fixes or upset the apple-cart with a 2.25 rebase, just disable
building gold for now.  It's never been officially supported for the
direct-to-native NaCl toolchains anyway.

BUG= none
TEST= toolchain trybots
R=dschuff@chromium.org

Committed: https://chromium.googlesource.com/native_client/src/native_client/+/4dbae347415d2448b5418db3439d832b8266893a

[Derek Schuff, 2015-11-18 23:26:00]

lgtm

[Roland McGrath, 2015-11-19 00:12:37]

Committed patchset #1 (id:1) manually as
4dbae347415d2448b5418db3439d832b8266893a (presubmit successful).