ff_square_tab is always used with an offset; if this table
is marked as hidden, the compiler can infer that it and
therefore also ff_square_tab + 256 have a fixed offset
from the code. This allows to avoid performing "+ 256"
at runtime by baking it into the offset from the code to
the table.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>