GitLab

- 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

Review URL: https://codereview.chromium.org/562383002

Cr-Commit-Position: refs/heads/master@{#294489}