GN: Refactor PNaCl toolchain definitions to use a template

GN: Refactor PNaCl toolchain definitions to use a template

This reduces duplicated code by using a template.
Ironically, some extra duplication is necessitated by GN bug 555724.
The variable definition can be hoisted back up when that bug is fixed.

BUG= none
R=dpranke@chromium.org, phosek@chromium.org

Committed: https://crrev.com/8b3b31d64338c7eadb59b433da6a933f9ad1c0ca
Cr-Commit-Position: refs/heads/master@{#359643}

[Petr Hosek, 2015-11-13 20:40:55]

lgtm

[Dirk Pranke, 2015-11-13 20:50:28]

lgtm

[Roland McGrath, 2015-11-13 21:01:32]

The CQ bit was checked by mcgrathr@chromium.org

[commit-bot: I haz the power, 2015-11-13 21:05:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1450453002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1450453002/1

[commit-bot: I haz the power, 2015-11-13 21:16:02]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-13 21:17:13]

Patchset 1 (id:??) landed as
https://crrev.com/8b3b31d64338c7eadb59b433da6a933f9ad1c0ca
Cr-Commit-Position: refs/heads/master@{#359643}