Addresses multiple issues in the arm context save/restore code:
-   When restoring from context, do not clobber pointers in CPUARMState
    with data from the context buffer.  This causes use-after-free bugs
    usually manifesting as double-free crashes.
-   When saving to context, do not copy pointers from CPUARMState to the
    context buffer, to avoid leaking pointers to the context.  Instead,
    zero the respective area of the context buffer, to ensure that
    reg_read and reg_write can access fields outside of the copied
    range.  Specifically, env->uc needs to be NULL when accessed for a
    context.
-   When restoring to a CPU with a different nr than the source CPU,
    copy min(nr, ctx_nr) instead of nothing at all.
-   Add regression tests checking all CPU models for arm and arm64 for
    trivial double-free crashes.

Fixes unicorn-engine#2195