memtester: Try to guess if it was a read or write operation failing

Basically we just compare the buffers twice. And if the result is
reproducible, then it is likely that a corrupted value is already
in DRAM as a result of writing earlier. And if the result of
comparing buffers in not reproducible, then it was a read error
that happened during the compare operation itself.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
memtester: Optimize compare_regions for ARM NEON

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
memtester: Test program for compare_regions_helper_* implementations

May be useful to verify the correctness of ARM NEON or maybe some
other optimizations.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
memtester: Added support for MEMTESTER_SKIP_STUCK_ADDRESS env variable

The stuck address test is not very useful when trying different
dram_tpr3 settings, but takes time and can could not be disabled
before. Now we can skip it.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>