Fix missing used attribute for inline assembly variables

Variables used in inline assembly need to be marked with attribute((used)).
Static constants already were, via the define of DECLARE_ASM_CONST.
But DECLARE_ALIGNED does not add this attribute, and some of the variables
defined with it are const only used in inline assembly, and therefore
appeared dead. This change adds a macro DECLARE_ASM_ALIGNED that marks
variables as used.

This change makes FFMPEG work with Clang's ThinLTO.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>