Refactor NaCl IRT toolchains, use arm-nacl-clang

Refactor NaCl IRT toolchains, use arm-nacl-clang

This factors the irt_* toolchain definitions using a template to reduce
copied boilerplate.  It also switches the ARM build to use arm-nacl-clang
rather than arm-nacl-gcc, which matches what the GYP build does now.  Many
of the compilation flags are still not at all right, but this is enough
for the build of the IRT to complete successfully.

BUG=531702
R=dpranke@chromium.org

Committed: https://crrev.com/916aafa6563eb600b60a683d52d0c121f2eaee92
Cr-Commit-Position: refs/heads/master@{#348781}

[Dirk Pranke, 2015-09-14 22:22:27]

lgtm. 

this all looks correct.

[Roland McGrath, 2015-09-14 22:22:59]

The CQ bit was checked by mcgrathr@chromium.org

[commit-bot: I haz the power, 2015-09-14 22:24:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1341143002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1341143002/1

[commit-bot: I haz the power, 2015-09-15 00:06:59]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-15 00:07:49]

Patchset 1 (id:??) landed as
https://crrev.com/916aafa6563eb600b60a683d52d0c121f2eaee92
Cr-Commit-Position: refs/heads/master@{#348781}

[commit-bot: I haz the power, 2015-09-23 12:39:50]

Patchset 1 (id:??) landed as
https://crrev.com/916aafa6563eb600b60a683d52d0c121f2eaee92
Cr-Commit-Position: refs/heads/master@{#348781}