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

UltraBlame original commit: 62adf4084bae5e2f6d49cd3c4c6cbfbc2691a2aa

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

@@ -107,28 +107,34 @@ rustc
 .
 15
 .
-0
+1
+-
+dev
 (
-10893a9a3
+38014d1b6
 2017
 -
-01
+02
 -
-19
+08
 )
-repack
+gecko
+build
+with
+-
+fPIC
 "
 "
 size
 "
 :
-110076708
+75614428
 "
 digest
 "
 :
 "
-2c865f12279b103e8861071e05480cd8aeb9c4e4cd63eea1b8ca50fb92880583bebd27a3af6a86b3f12b9ee89e70839140f061ab829fcceca5e85dc8bc428ec3
+1bfd06db51c4aeaf43e43d8080069bba4b3d590a4863366ceb9c9a9915fbc528111fa067a13fc701ee00cbeda21a9f5de23d20d2479eab35fa2d7f729c660159
 "
 "
 algorithm