[lldb][AArch64] Read mte_ctrl register from core files (#69689)

This register reports the configuration of the AArch64 Linux tagged
address ABI, part of which is the memory tagging (MTE) settings.

It will always be present in core files because even without MTE, there
are parts of the tagged address ABI that can be configured (these parts
use the Top Byte Ignore feature).

I missed adding this when I previously worked on MTE support. Until now
you could read memory tags from a core file but not this register.