gn: various flag matching for 'base'

gn: various gyp flag matching for 'base'

- Don't remove -fno-ident from gyp build (was added about 3y ago, seemingly unnecessary for current clang)
- Add -fno-slp-vectorize to GN to make builds match (both to be removed after next clang roll)
- Make usage of -Wno-reserved-user-defined-literal match (required for dbus pre-trusty)
- Add -march=x86-64 to gn cflags
- Default symbol_level based on is_debug, and turn it off by default in Release (same as gyp)

Also, fiddle with output of gyp_flag_compare.py.

R=brettw@chromium.org
BUG=335824
Committed: https://crrev.com/29682b848742042388ebd29c9df775f074c1414c
Cr-Commit-Position: refs/heads/master@{#294489}

[scottmg, 2014-09-11 20:24:29]

scottmg@chromium.org changed reviewers:
- thakis@chromium.org

[scottmg, 2014-09-11 20:32:24]

Patchset #3 (id:40001) has been deleted

[scottmg, 2014-09-11 20:32:29]

Patchset #2 (id:20001) has been deleted

[scottmg, 2014-09-11 20:37:53]

Patchset #2 (id:60001) has been deleted

[scottmg, 2014-09-11 22:06:25]

scottmg@chromium.org changed reviewers:
+ thakis@chomium.org

[scottmg, 2014-09-11 22:06:44]

thakis: As punishment for being helpful you've been upgraded to a full reviewer!

[scottmg, 2014-09-11 22:07:03]

(for BUILDCONFIG.gn)

[brettw, 2014-09-11 22:49:12]

lgtm

[scottmg, 2014-09-11 22:49:43]

The CQ bit was checked by scottmg@chromium.org

[commit-bot: I haz the power, 2014-09-11 23:22:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/562383002/180001

[scottmg, 2014-09-11 23:29:05]

Committed patchset #7 (id:180001) to pending queue manually as fc54cb5
(presubmit successful).

[commit-bot: I haz the power, 2014-09-11 23:32:13]

Patchset 7 (id:??) landed as
https://crrev.com/29682b848742042388ebd29c9df775f074c1414c
Cr-Commit-Position: refs/heads/master@{#294489}