GN: Factor CPU/ABI and code-generation options out of main :compiler config

GN: Factor CPU/ABI and code-generation options out of main :compiler config

This factors out two subconfigs from the main "compiler" config, which
now incorporates them by reference.  Other than the order of some
options to the compiler, this should not change anything about the build
(except for x86-32 NaCl, see below).

The first is "compiler_cpu_abi", which contains the basic options for
selecting the target CPU and ABI for the compiler.  This is useful for
cases like nacl_helper_bootstrap, which use the same compiler and the
same target-selection options, but do not want everything else that
"compiler" pulls in.

The second is "compiler_codegen", which contains the specific options
for tweaking code generation.  This clarifies which options are for
which purposes, and separates the conditional blocks so one would not be
tempted to throw a random code generation tweak into the generic target
selection options, where one could easily overlook issues like whether
the option ought to apply to NaCl compilers.  To wit, all the existing
code generation tweaks in this subconfig are not applied to NaCl, which
they never should have been.

The options in these topical categories that are already separated into
the //build/config/<OS>:compiler subconfigs are not factored out.

BUG= none
R=dpranke@chromium.org

Committed: https://crrev.com/a25ad97b2ac0a01a394db584b2fc9bb5378b1b63
Cr-Commit-Position: refs/heads/master@{#353111}

[Dirk Pranke, 2015-10-08 01:36:20]

lgtm

[Roland McGrath, 2015-10-08 17:21:14]

The CQ bit was checked by mcgrathr@chromium.org

[commit-bot: I haz the power, 2015-10-08 17:21:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1393143003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1393143003/1

[commit-bot: I haz the power, 2015-10-08 18:47:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-08 18:48:32]

Patchset 1 (id:??) landed as
https://crrev.com/a25ad97b2ac0a01a394db584b2fc9bb5378b1b63
Cr-Commit-Position: refs/heads/master@{#353111}