CI: misc cleanups + faster analysis (#449)

* switch to git ls-files to avoid picking up any other local .c files
* enable assertions during static analysis since we used some assertions to
  disable/silence certain warnings.
* update TCC commit hash to a more recent one
* parallelize static analysis

cppcheck already has -j argument to parallelize it's analysis and
provide results faster, clang-tidy unfortunately doesn't.

so use xargs -P to archive parallel execution. on my system this brings
down the analysis time from ~27s to ~5s.

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