GN: Refactor Linux Clang toolchain definitions, plumb use_gold for toolchain_args()

GN: Refactor Linux Clang toolchain definitions, plumb use_gold for toolchain_args()

This factors the commonalities of defining Clang toolchains into a
new "clang_toolchain" template.  As well as reducing boilerplate in
build/toolchain/linux/BUILD.gn, this makes it more straightforward
to define variant toolchains the diverge slightly from the standard
Clang toolchains.  One such divergence needed for the case of a
special toolchain to handle nacl_helper_bootstrap is the use_gold
setting; so that is plumbed through the gcc_toolchain template, and
the new clang_toolchain template, such that their invokers can get
their use_gold setting into the toolchain_args() block.

To make toolchain_args() overriding of use_gold affect the default
values of declare_args() variables that depend on use_gold, those
initializations are moved out of the declare_args() block.

BUG= none
R=dpranke@chromium.org

Committed: https://crrev.com/de55ff1d33fcf3b5312034542d642b54766a25be
Cr-Commit-Position: refs/heads/master@{#353081}

[Dirk Pranke, 2015-10-08 01:27:09]

dpranke@chromium.org changed reviewers:
+ brettw@chromium.org

[Dirk Pranke, 2015-10-08 01:27:09]

lgtm. 

You might find Brett to be a more suitable reviewer for this stuff than I; I
think he tends to have stronger opinions as to how this stuff is structured at
least.

[Roland McGrath, 2015-10-08 17:06:16]

The CQ bit was checked by mcgrathr@chromium.org

[commit-bot: I haz the power, 2015-10-08 17:07:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1390383002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1390383002/1

[commit-bot: I haz the power, 2015-10-08 17:14:20]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-08 17:15:33]

Patchset 1 (id:??) landed as
https://crrev.com/de55ff1d33fcf3b5312034542d642b54766a25be
Cr-Commit-Position: refs/heads/master@{#353081}