crypto: chacha20: always use our fallback timingsafe_bcmp rather than libc's

[theuni]

Looking at libc sources, apple and openbsd implementations match our naive fallback. Only FreeBSD (and only x86_64) seems to implement an optimized version.

It's not worth the hassle of using a platform-specific function for such little gain.

Additionally, as mentioned below, this is the only case outside of sha2 that requires an autoconf check, and I have upcoming PRs to remove the sha2 ones.

Apple's impl is unoptimized.

As-is OpenBSD's impl.

Relevant IRC conversation with sipa:

<cfields> sipa: chacha20poly1305.cpp uses libc's timingsafe_bcmp when possible. But looking around at apple/freebsd/openbsd, I don't see any impl that doesn't use the naive implementation that matches our fallback...
<cfields> is there any reason to belive there's an optimized impl somewhere that we're actually hitting?
<cfields> asking because after cleaning up sha2, timingsafe_bcmp is the last autoconf check that remains in all of crypto. It'd make life easy if we could just always use our internal one.
<cfields> *all of crypto/
<sipa> cfields: let's get rid of the dependency then
<sipa> it's a trivial function
<sipa> and if we need it for some platforms, no real reason not to use it on all

After the above discusstion, I did end up finding the x86_64-optimized FreeBSD impl, but I don't think that's all that significant.

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[theuni]

Ping @sipa for a quick concept ACK.

[sipa]

utACK 2d18194

[fanquake]

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[hebasto]

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[theStack]

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[fanquake]

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[TheCharlatan]

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[theStack]

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As a historical side-note, it seems like this function was first introduced for OpenSSH (managed in the OpenBSD tree) in 2010, with the name timing_safe_cmp back then: openbsd/src@8488487#diff-1baa12ad01bad68b45e89594ef3309ad070f9848f764f976e08c435ade846ae5R833