linux: make fstatat(2) handle AT_EMPTY_PATH

Without it, Qt5 apps from Focal fail to start, being unable to load
their plugins. It's also necessary for glibc 2.33, as found in recent
Arch snapshots.

PR: 254112
Reviewed By: kib
Sponsored by: The FreeBSD Foundation, EPSRC
Differential Revision: https://reviews.freebsd.org/D28192