util: use HAVE_FDATASYNC to determine fdatasync() use

Summary:

Rather than just using it on Linux and NetBSD, use fdatasync() based
on whether it's available. i.e it is available in newer versions
of FreeBSD (11.1 and later).

This also aligns our code more closely with what is being done in leveldb.

Was pointed out by Luke in #19430.

This is a backport of core#19614

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10060