Bump gmp-tarballs to fix #22497

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 (closed).

Additionally, this includes a backport of the fix for CVE-2021-43618, fixing #22789 (closed).