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/include/tcg/tcg.h

@@ -709,6 +709,7 @@ struct TCGContext {
     TCGv_i64 cpu_bndu[4];
 
     /* qemu/tcg/i386/tcg-target.inc.c */
+    /* qemu/tcg/aarch64/tcg-target.inc.c */
     void *tb_ret_addr;
 
     /* target/riscv/translate.c */