Several bugfixes (#2049)

* Remove global variable from aarch64 tcg target

This obviously breaks trying to run two unicorn instances at once on
aarch64. It appears a similar variable had already been moved to the
state struct for i386 tcg target.

* Reenable writing to jit region while calling tb_add_jump

On arm macs, every place that writes to jit code needs to have
tb_exec_unlock called first. This is already in most necessary places,
but not this one.

* Don't forget to call restore_jit_state in uc_context_restore

Every time UC_INIT is used, restore_jit_state must be used on the return
path, or occasional assertion failures will pop up on arm macs.

* Restore pc before calling into tlb fill hook

In my application it is important to have correct pc values available
from this hook.

qemu/softmmu/unicorn_vtlb.c

@@ -55,6 +55,8 @@ bool unicorn_fill_tlb(CPUState *cs, vaddr address, int size,
     struct hook *hook;
     HOOK_FOREACH_VAR_DECLARE;
 
+    cpu_restore_state(cs, retaddr, false);
+
     HOOK_FOREACH(uc, hook, UC_HOOK_TLB_FILL) {
         if (hook->to_delete) {
             continue;