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Marvell PJ4 core used in CuBox very poorly handles VFP uncached
reads from the framebuffer. Using WMMX or ARM LDM reads is much
faster, with LDM instructions having a minor advantage. This
improves framebuffer read performance from ~50MB/s to ~100MB/s.

WMMX runtime detection and PJ4 core identification is also added
as part of this fix.
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>