target/arm: Don't advertise aarch64-pauth.xml to gdb

target/arm: Don't advertise aarch64-pauth.xml to gdb

Unfortunately a bug in older versions of gdb means that they will
crash if QEMU sends them the aarch64-pauth.xml.  This bug is fixed in
gdb commit 1ba3a3222039eb25, and there are plans to backport that to
affected gdb release branches, but since the bug affects gdb 9
through 12 it is very widely deployed (for instance by distros).

It is not currently clear what the best way to deal with this is; it
has been proposed to define a new XML feature name that old gdb will
ignore but newer gdb can handle.  Since QEMU's 8.0 release is
imminent and at least one of our CI runners is now falling over this,
disable the pauth XML for the moment.  We can follow up with a more
considered fix either in time for 8.0 or else for the 8.1 release.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>