tests/qtest/migration-test: Use custom asm bios for ppc64

Similar to other archs, build a custom bios memory updater. Running the
test with OF code is a cool trick, but SLOF takes a long time to boo

tests/qtest/migration-test: Use custom asm bios for ppc64

Similar to other archs, build a custom bios memory updater. Running the
test with OF code is a cool trick, but SLOF takes a long time to boot.
This reduces test time by around 3x (150s to 50s).

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

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