[libcxx] Re-include <osreldate.h> in __config for FreeBSD

In 0a97720d0197 some changes were made to `__config` for assuming that
`__BYTE_ORDER__` is always present.

However, this deleted a `<osreldate.h>` include for FreeBSD, which is
required to get the value of `__FreeBSD_version`, and that is used later
in the file to determine whether `_LIBCPP_C_HAS_NO_GETS` needs to be
enabled.

Include `<osreldate.h>` just after the other includes used for feature
detection, to fix this.

Note that when FreeBSD 13 is EOLed, this can be removed, as then all
supported FreeBSD versions will no longer have `gets()`.