hevcdec: Use LOCAL_ALIGNED_* for declaring local variables with alignment

Not all compilers can do alignment larger than the normal stack alignment
for variables on the stack. In these cases, the LOCAL_ALIGNED_* macros
produce the workaround alignment wrapper consisting of a padded array
and a pointer variable.

This fixes the hevc fate tests on RVCT/ARMCC after adding IDCT assembly
that actually assumes/relies on this alignment.

Signed-off-by: Martin Storsjö <martin@martin.st>