[lldb][AArch64] Read SME2's ZT0 register from Linux core files (#70934)

The ZT0 register is always 64 bytes in size so it is a lot easier to
handle than ZA which is scalable. In addition, reading an inactive ZT0
via ptrace returns all 0s, unlike ZA which returns no register data.

This means that a corefile from a process where ZA and ZT0 were inactive
still contains an NT_ARM_ZT note and we can simply say that if it's
there, then we should be able to read from it.

Along the way I removed a redundant check on the size of the ZA note. If
that note's size is < the ZA header size, we do not have SME, and
therefore could not have SME2 either.

I have added ZT0 to the existing SME core files tests. This means that
you need an SME2 system to generate them (Arm's FVP at this point). I
think this is a fair tradeoff given that this is all running in
simulation anyway and seperate ZT0 tests would be 99% identical copies
of the ZA only tests.