PR standards/44921: Add errno consts for robust mutexes

Add the two missing errno.h constants: EOWNERDEAD and ENOTRECOVERABLE.
While technically they're used for robust mutexes which we do not
support at the moment, they are listed in POSIX and used by libc++.
While libc++ can be made to build without it, it just locally redefines
the values then, so we may as well define them globally.

(mgorny)