Alas, this involved quite a bit of copy-n-paste between the
architectures, but this is caused by the very convoluted
relationships, lifetimes and distribution of responsibilities. This
should really be cleaned up by moving code around and using STL maps,
but that's not really a priority right now.

Bonus: Fixed leaks in the ARM64 disassembler tests.

Review URL: https://codereview.chromium.org/1132943007

Cr-Commit-Position: refs/heads/master@{#28496}