This reverts commit 1a17e17c462bdd4e1d26d8742f8b7087273e04c2.

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mbed-os is a directory that gets checked out by the mbed cli
(a tool similar in spirit to miot) and it contains a lot of
.c sources, so many of them that clang-format burns your CPU.

While we're at it, also fix check_no_ifdefs when running under a commit
hook.

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Identifiers starting with _ are reserved, fix those.
Wrote a tool to do it and applied it globally, why not.

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