change QARMA3 default for aarch64?

[Michael Tokarev]

Since commit v8.1.0-511-g399e5e7125 "target/arm: Implement FEAT_PACQARMA3",
pauth-qarma3 is the default pauth scheme.  However this one is very slow.

When people run aarch64 code in qemu tcg, an immediate reaction is like,
"this seems to be a bug somewhere", since the code run insanely slower than
it was before.

And this is very difficult to find as well, - the reason for that slowdown
is usually well hidden from an average soul.

When the reason is actually discovered, people start changing settings in
various tools and configs to work around this issue.  Qemu itself has
overrides, pauth-impdef=on, in various tests, to make the test run at
saner speed.

After seeing how many issues people are having in debian with that, I'm
about to switch the default in debian build of qemu, because impdef,
while makes certain arm64-specific protection feature less effective,
is actually significantly more practical.  I dislike changing the
defaults, but this is a situation when it needs to be done, imho.

But before doing that, maybe it's better to change qemu default
instead?  What do you think?

Ditto for QARMA5 I guess.

/mjt