linux: Partially implement TCSBRK

This fixes tcflush(3), unbreaking cheribuild.py under arm64 Focal.

Reviewed By: imp
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D32291

(cherry picked from commit 7e7859e7c2b900fd1b1af6e90ed31d51b9b5514d)