Fix register scanning on FreeBSD and Linux#38

Open

gareth-rees wants to merge 2 commits intomasterfrom

branch/2020-09-01/regscan

Commits on Sep 3, 2020

Restore assembly code for spilling callee-save registers.
gareth-rees
committed

Commits on Jan 16, 2023

Catch-up merge master into branch/2020-09-01/regscan to get full CI build and test, and prepare for review and merge.
rptb1
committed