syscall: enforce minimum buffer size to call ReadDirent

freebsd and netbsd require a minimum buffer size of 1K.

Note this doesn't quite fix freebsd, it has other bugs,
I'll file a separate issue.

Fixes #31403

Change-Id: I9d7e78f6d30859b34715afadc4b8bd3b1ecc606b
Reviewed-on: https://go-review.googlesource.com/c/go/+/171757


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