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}