Bug 1336155 - Update linux32 rust builders to use -fPIC. r=mshal

Use a custom build of rust 1.15.1 with an additional bump of the
gcc crate to 0.3.43 to pass -fPIC to the C compiler on i686-linux.

While 1.15.1 was tagged today, there's some question as to whether
it would be released from the tag or if the tag would be moved
to incorporate this fix.

This works around the issue with text segment relocations with
the 1.15.0 stable release. For more information see the upstream
issue at rust-lang/rust#39523

MozReview-Commit-ID: 83IxtJeJxlh

--HG--
extra : rebase_source : 8deda9058c77a98c65a46949768702c56296d9d4

browser/config/tooltool-manifests/linux32/releng.manifest

@@ -16,9 +16,9 @@
 "unpack": true
 },
 {
-"version": "rustc 1.15.0 (10893a9a3 2017-01-19) repack",
-"size": 110076708,
-"digest": "2c865f12279b103e8861071e05480cd8aeb9c4e4cd63eea1b8ca50fb92880583bebd27a3af6a86b3f12b9ee89e70839140f061ab829fcceca5e85dc8bc428ec3",
+"version": "rustc 1.15.1-dev (38014d1b6 2017-02-08) gecko build with -fPIC",
+"size": 75614428,
+"digest": "1bfd06db51c4aeaf43e43d8080069bba4b3d590a4863366ceb9c9a9915fbc528111fa067a13fc701ee00cbeda21a9f5de23d20d2479eab35fa2d7f729c660159",
 "algorithm": "sha512",
 "filename": "rustc.tar.xz",
 "unpack": true