[lldb][AArch64] Fix corefile memory reads when there are non-address bits

Previously if you read a code/data mask before there was a valid thread
you would get the top byte mask. This meant the value was "valid" as in,
don't read it again.

When using a corefile we ask for the data mask very early on and this
meant that later once you did have a thread it wouldn't read the
register to get the rest of the mask.

This fixes that and adds a corefile test generated from the same program
as in my previous change on this theme.

Depends on D118794

Reviewed By: omjavaid

Differential Revision: https://reviews.llvm.org/D122411