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/accel/tcg/cpu-exec.c

@@ -288,7 +288,9 @@ static inline TranslationBlock *tb_find(CPUState *cpu,
     }
     /* See if we can patch the calling TB. */
     if (last_tb) {
+        tb_exec_unlock(cpu->uc);
         tb_add_jump(last_tb, tb_exit, tb);
+        tb_exec_lock(cpu->uc);
     }
 
     return tb;