Bug 1338061 - Move toolchain tasks to a separate "platform". r=dustin

The toolchain tasks are hard to spot on treeherder, in the ocean of
build and test jobs associated with the platforms they are currently
under.

Now that we have a significant number of toolchain tasks across
different platforms, it's even worse, especially combined with the fact
that they don't happen on every push.

To make them more easily visible, we move them to a new, separate,
"platform", with the name "toolchains", instead of having them in
different platforms. But since the distinction between Linux, OSX and
Windows 32/64 is still interesting to have, we create groups for each of
those platforms.

But because of bug 1215587, the jobs still end up associated to their
previous group, defeating the new grouping, so to work around that bug,
we also rename the jobs in subtle ways.

taskcluster/taskgraph/transforms/task.py

@@ -366,7 +366,10 @@ GROUP_NAMES = {
     'tc-BMcs': 'Beetmover checksums, executed by Taskcluster',
     'Aries': 'Aries Device Image',
     'Nexus 5-L': 'Nexus 5-L Device Image',
-    'Cc': 'Toolchain builds',
+    'TL': 'Toolchain builds for Linux 64-bits',
+    'TM': 'Toolchain builds for OSX',
+    'TW32': 'Toolchain builds for Windows 32-bits',
+    'TW64': 'Toolchain builds for Windows 64-bits',
     'SM-tc': 'Spidermonkey builds',
 }
 UNKNOWN_GROUP_NAME = "Treeherder group {} has no name; add it to " + __file__