[rebase] threads: fix unitialized members in sched_param

Summary:
Rebased theuni's #14342.

Backport of Bitcoin Core PR14839
https://github.com/bitcoin/bitcoin/pull/14839

Test Plan:

make check

Reviewers: Fabien, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4008