xen/arm: set CPSR Z bit when creating aarch32 guests

xen/arm: set CPSR Z bit when creating aarch32 guests

The first 32 bytes of zImage are NOPs. When CONFIG_EFI is enabled in the
kernel, certain versions of Linux will use an UNPREDICTABLE NOP
encoding, sometimes resulting in an unbootable kernel. Whether the
resulting kernel is bootable or not depends on the processor. See commit
a92882a4d270 in the Linux kernel for all the details.

All kernel releases starting from Linux 4.9 without commit a92882a4d270
are affected.

Fortunately there is a simple workaround: setting the "Z" bit in CPSR
make it so those invalid NOP instructions are never executed. That is
because the instruction is conditional (not equal). So, on QEMU at
least, the instruction will end up to be ignored and not generate an
exception. Setting the "Z" bit makes those kernel versions bootable
again and it is harmless in the other cases.

Note that both U-Boot and QEMU -kernel set the "Z" bit in CPSR when
booting a zImage kernel on aarch32.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Wei Chen <Wei.Chen@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>