target/arm: Limit ID register assertions to TCG

In arm_cpu_realizefn() we make several assertions about the values of
guest ID registers:
 * if the CPU provides AArch32 v7VE or better it must advertise the
   ARM_DIV feature
 * if the CPU provides AArch32 A-profile v6 or better it must
   advertise the Jazelle feature

These are essentially consistency checks that our ID register
specifications in cpu.c didn't accidentally miss out a feature,
because increasingly the TCG emulation gates features on the values
in ID registers rather than using old-style checks of ARM_FEATURE_FOO
bits.

Unfortunately, these asserts can cause problems if we're running KVM,
because in that case we don't control the values of the ID registers
-- we read them from the host kernel.  In particular, if the host
kernel is older than 4.15 then it doesn't expose the ID registers via
the KVM_GET_ONE_REG ioctl, and we set up dummy values for some
registers and leave the rest at zero.  (See the comment in
target/arm/kvm64.c kvm_arm_get_host_cpu_features().) This set of
dummy values is not sufficient to pass our assertions, and so on
those kernels running an AArch32 guest on AArch64 will assert.

We could provide a more sophisticated set of dummy ID registers in
this case, but that still leaves the possibility of a host CPU which
reports bogus ID register values that would cause us to assert.  It's
more robust to only do these ID register checks if we're using TCG,
as that is the only case where this is truly a QEMU code bug.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20190718125928.20147-1-peter.maydell@linaro.org
Fixes: https://bugs.launchpad.net/qemu/+bug/1830864
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

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