[libc++] Add FreeBSD-specific EINTEGRITY errc value

Since ~2019 we have been carrying a libc++ patch in FreeBSD, that adds
the `EINTEGRITY` errno value. At the time, I submitted
https://reviews.llvm.org/D56398 for this, but due to the complicated
`ELAST` handling in `errno.h`, which that patch made even more
complicated, it was put on hold.

I would like to try this again, but much simplified: there is no need to
modify `errno.h`, since all supported versions of FreeBSD define both
`EOWNERDEAD` and `ENOTRECOVERABLE`, so the part that attempts to
redefine `ELAST` is effectively never used, and for FreeBSD we can
simply ignore it.

The only proposed changes will now be to optionally add `EINTEGRITY` as
an `enum errc` value, and add a corresponding test to `errc.pass.cpp`.