classes/qemu: use tune to select QEMU_EXTRAOPTIONS, not package architecture

classes/qemu: use tune to select QEMU_EXTRAOPTIONS, not package architecture

Using the package architecture to select the right qemu options to pass
to qemu-user is incorrect, and fails for recipes that set PACKAGE_ARCH
to MACHINE_ARCH (as the qemuppc workarounds suggest) because there are
not typically any options set for the machine name.

Solve this by using TUNE_PKGARCH instead: for the majority of recipes
this is the same value, but for machine-specific recipes it remains the
same instead of changing to the machine name.

This means we can remove the qemuppc workarounds, as they're obsolete.

Also update the gcc-testsuite recipe which uses the same pattern to use
TUNE_PKGARCH, and generalise the else codepath to avoid needing to
update the list of architectures.

[ YOCTO #15647 ]

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>