[libc] fix more readability-identifier-naming lints (#83914)

Found via:

    $ ninja -k2000 libc-lint 2>&1 | grep readability-identifier-naming

Auto fixed via:

    $ clang-tidy -p build/compile_commands.json \
      -checks="-*,readability-identifier-naming" \
      <filename> --fix

This doesn't fix all instances, just the obvious simple cases where it makes
sense to change the identifier names.  Subsequent PRs will fix up the
stragglers.