arm: Mark the stack as non-executable

If linking in an object file without this attribute set, the
linker will assume that an executable stack might be needed.

Signed-off-by: Martin Storsjö <martin@martin.st>
movenc: Add a fallback fragmentation method for plain mp4 as well

Previously the default fragmentation method was only enabled
if writing an ISM file.

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