[lldb][AArch64] Fix tagged watch test on Graviton 3

During __do_global_dtors_aux glibc sets a flag that is right
next to the global variable. This is done using a store byte.

On QEMU the watchpoints are handled with a finer granularity
than real hardware, so this wasn't a problem. On Graviton 3
(and Mountain Jade, though this test won't run there) watchpoints
look at larger chunks of memory.

This means that the final continue actually stops in  __do_global_dtors_aux
instead of exiting.

We could fix this by padding the global to be away from the flag,
but that is fiddly and it is easier just to remove the watchpoint
before the final continue. We have already verified it worked by that
point.

Reviewed By: omjavaid

Differential Revision: https://reviews.llvm.org/D154201