[lldb][AArch64] Add the tpidr2 TLS register that comes with SME

This changes the TLS regset to not only be dynamic in that it could
exist or not (though it always does) but also of a dynamic size.

If SME is present then the regset is 16 bytes and contains both tpidr
and tpidr2.

Testing is the same as tpidr. Write from assembly, read from lldb and
vice versa since we have no way to predict what its value should be
by just running a program.

Reviewed By: omjavaid

Differential Revision: https://reviews.llvm.org/D154930