Attributes: replace custom void* union with C union

The current implementation uses a void* to store different types of
attribute value integers and attempts to figure out proper offsets
for storing smaller integers in that pointer. The required pointer
aliasing is UB and causes issues with GCC 11.

The new implementation replaces the self-built pointer-based union
with a C union and selects the (pointer to the) right field based
on the av_set_from value.

This patch also fixes a bug where copied attributes always had the
set_from field set to C pointer, which worked but is technically not
correct.

Signed-off-by: Joseph Schuchart <[email protected]>

Author: devreal