Simplify the Bazel-related CI scripts and BUILD configuration in
preparation for adding several more for the compression feature.

- Move CC=clang to .bazelrc

- Define CXX in case it's already defined in the environment, in
  which case it overrides deferring to CC

- Replace --repo_env, which doesn't affect build, with --action_env

- Move commented-out ubsan invocation to its own CI job. Since
  ubsan was commented out, Bazel invocations have moved to their
  own jobs so they can run in parallel.

- Consistently test all targets, `...`, since TFLM is no longer
  confined to the tensorflow/lite/micro directory. Filter out
  inappropriate new targets with no.san tags.

- Don't run `bazel build ...` before `bazel test ...`; the latter
  already builds all non-test targets.

- Remove obsolete filter for `no_oss`, there were no more
  `no_oss` tags in the tree.

- Stop using readable_run: it has diverged from its original
  meaning and now only redirects stderr to stdout. This is not
  needed.

BUG=see description

Clarify that the scripts run from the root directory of the
project by restoring the change of directory at the top of the
script, even though CI runs from the root directory of the
project.

Also clarify the intention that all tests in the project be run
by specifying `//...`, rather than `...`, which means all targets
at or below the working directory.