This is to help diagnose #25498. We can't use `unexpectedErrno` here,
because `std.posix.munmap` is infallible. So, when the flag is set to
report unexpected errnos, we just call `std.debug.panic` to provide
details instead of doing `unreachable`.

Pushing straight to master after running checks locally; there's no
point waiting for CI on the PR just for this.