Unlike x86, ARM doesn't support a generic thread pointer for TLS data,
so things like
```
reg read tp
...
memory read tp
```
Don't work, and you need to specify tpidr. This works, especially
because that's the name GDB uses. But for ease of use, and at the
request of @aperez I've made it so we can reference it via `tp`.
I personally don't have an aarch machine, and all the arm examples in
`Shell/Register/Core` are freebsd and don't contain tpidr, so I was
unable to add a shell test for this. I added a test to the AARCH
register tests, but without an Aarch machine I'm hoping these work.
6505bc3db0