arm7_9: Fix broken halfword/byte memory reads

arm7_9: Fix broken halfword/byte memory reads

Always scan out all bits, but make sure only the allowed number of bytes
end up in the caller-provided buffer. Discard the rest by adding another
scan field when size < 4.

Rewrite the endianness callback to avoid reading outside allocated memory.
Make it directly usable as a callback without the need for a wrapper. Move
the shared callback to a more suitable home in arm7_9_common.

This fixes the regressions introduced in commits
991ed5a2b657e660f744eefddb084724e52938ea
cb90d32e386a7489d31136997209c61e9559ff5e
and
c3074f377c1da33ca8ba8493826e1b52351eebc6

Change-Id: Ia8bde8c5a9844e89a1d6c0bc8534cd26f02f8d11
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/789
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>