Bug 1175359 - Enable rust in linux64 automation builds. r=mshal

Write a mozconfig.rust fragment which makes the rust toolchain
provided by tooltool available for linux builds, similar to
what we do for MacOS X.

Include this in linux64 mozconfigs to enable rust for official
nightly builds of that target. These aren't used outside of automation
builds, so including rust there will verify code on check-in
without requiring developers to install rust.

We must whitelist the mozconfig fragment to pass the consistency
check since we're not ready to let this feature ride the trains
to beta and release.

The tooltool reference is to a custom build of rustc 1.4
with --enable-rpath to avoid having to add the rustc lib
directory to LD_LIBRARY_PATH which somehow conflicts with
the gtk3 build we also install through tooltool.

It is also built with --enable-llvm-static-stdcpp on a
rust-buildbot dist docker image (centos:5 + script updates)
to avoid issues with GLIBCXX and GLIBC symbol versions.

browser/config/mozconfigs/linux64/debug

@@ -18,5 +18,6 @@ export MOZ_PACKAGE_JSSHELL=1
 
 ac_add_options --with-branding=browser/branding/nightly
 
+. "$topsrcdir/build/unix/mozconfig.rust"
 . "$topsrcdir/build/mozconfig.common.override"
 . "$topsrcdir/build/mozconfig.cache"

browser/config/mozconfigs/linux64/nightly

@@ -10,5 +10,6 @@ STRIP_FLAGS="--strip-debug"
 
 ac_add_options --with-branding=browser/branding/nightly
 
+. "$topsrcdir/build/unix/mozconfig.rust"
 . "$topsrcdir/build/mozconfig.common.override"
 . "$topsrcdir/build/mozconfig.cache"

browser/config/mozconfigs/whitelist

@@ -40,6 +40,7 @@ whitelist['nightly']['linux64'] += [
     'STRIP_FLAGS="--strip-debug"',
     'ac_add_options --with-ccache=/usr/bin/ccache',
     '. "$topsrcdir/build/mozconfig.cache"',
+    '. "$topsrcdir/build/unix/mozconfig.rust"',
 ]
 
 whitelist['nightly']['macosx-universal'] += [

browser/config/tooltool-manifests/linux64/releng.manifest

@@ -19,5 +19,12 @@
 "algorithm": "sha512",
 "filename": "sccache.tar.bz2",
 "unpack": true
+},
+{
+"size": 73029932,
+"digest": "ef1818acf065838dcb72554e521f9fd7098f0a3690cb6a3106d7bf18f46c342bfdd5a2b7d86e92ee3ddb9e478380343e58ecf8fd242807b8881a2d53fbec5ab3",
+"algorithm": "sha512",
+"filename": "rustc.tar.xz",
+"unpack": true
 }
 ]

build/unix/mozconfig.rust

@@ -0,0 +1,9 @@
+# Options to enable rust in automation builds.
+
+# Tell configure to use the tooltool rustc.
+# Assume this is compiled with --enable-rpath so we don't
+# have to set LD_LIBRARY_PATH.
+RUSTC="$topsrcdir/rustc/bin/rustc"
+
+# Enable rust in the build.
+ac_add_options --enable-rust