syscall: fix Getdirentries on 32-bit freebsd 12

On freebsd 12, the system call for getdirentries writes 64 bits to
*basep, even on 32-bit systems. Accomodate that by providing a uint64
to the system call and copy the base to/from that uint64.
The uint64 seems to be a virtual file offset, so failing if the high
bits are not zero should be fine for reasonable-sized directories.

Fixes #32498

Change-Id: Ie22c0d301c6091bd20e813432928b24ab95cc314
Reviewed-on: https://go-review.googlesource.com/c/go/+/181377


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>