It turns out that gmp 6.2.1 uses the platform-reserved x18 register on
AArch64/Darwin. This was fixed in upstream gmp changeset 18164:5f32dbc41afc,
which was merged in 2020. Here I backport this patch although I do hope
that a new gmp release is forthcoming soon.
Fixes #22497.
Additionally, this includes a backport of the fix for CVE-2021-43618, fixing #22789 (closed).