[lldb][AArch64][Linux] Rename Is<ext>Enabled to Is<ext>Present (#70303)

For most register sets, if it was enabled this meant you could use it,
it was present in the process. There was no present but turned off
state. So "enabled" made sense.

Then ZA came along (and soon to be ZT0) where ZA can be present in the
hardware when you have SME, but ZA itself can be made inactive. This
means that "IsZAEnabled()" doesn't mean is it active, it means do you
have SME. Which is very confusing when we actually want to know if ZA is
active.

So instead say "IsZAPresent", to make these checks more specific. For
things that can't be made inactive, present will imply "active" as
they're never inactive.