Minimize libc++ errno-related header diffs with upstream

In commit 88640c0e8b6f5 the new EINTEGRITY errno value was added, and
this caused us to carry a patch for upstream libc++ since that time.
Because it can cause merge conflicts when importing libc++ code from
upstream, I have submitted an upstream pull request to get most of that
patch integrated.

It turns out that we do not need the errno.h part of it at all, since
all supported FreeBSD versions define EOWNERDEAD and ENOTRECOVERABLE,
and therefore the block that juggles with ELAST values is never used in
FreeBSD. At the moment it only applies to older versions of Linux, or
possibly other platforms.

Therefore the only part that needs to stay is the definition of a enum
errc value for EINTEGRITY, and this is made optional upon EINTEGRITY
being defined, to make it suitable for upstreaming.

No functional change is intended.

MFC after: 1 week

(cherry picked from commit 1ff41cad716adeba0d408652c92c81e59e3ba316)