Fixes errors that can lead to crashes in relocate_dynamorio, where its access to a function pointer and to the page size can both return garbage pre-relocation. We solve this with assembly to get the current PC, and just using a 4K minimum page size for the backward walk.
Tested with the PR #5462 branch where the new client.attach_blocking test crashes without this fix on AArch64. Manually tested on arm as well.
Fixes #5463 (closed)