Fix more aspects of the NaCl build configs.
As part of the un-forking of the NaCl GN build configs, we need
to be a little more careful about which NaCl toolchains are considered
to be "clang" or not and whether some of the clang-specific compiler
settings need to be applied in the NaCl contexts.
Also, remove more stray references to the old Nacl build configs.
R=brettw@chromium.org, mcgrathr@chromium.org, thakis@chromium.org
BUG=433528
Review URL: https://codereview.chromium.org/
1307653008
Cr-Commit-Position: refs/heads/master@{#347262}