Use sysctl for seeding on MacOS/BSD

Summary:
Partial backport of core PR17270:
https://github.com/bitcoin/bitcoin/pull/17270/commits/a81c494b4c9a8c2f1a319a03375826f12863706f

Test Plan:

ninja all check-all

Build for OSX (native and cross build), check the binary runs.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6201