intreadwrite: Add intermediate variables in the byteswise AV_W*() macros

intreadwrite: Add intermediate variables in the byteswise AV_W*() macros

This avoids issues with expanding the argument multiple times,
and makes sure that it is of the right type for the following shifts.

Even if the caller of a macro could be expected not to pass parameters
that have side effects if expanded multiple times, these fallback
codepaths are rarely, if ever, tested, so it is expected that such
issues can arise.

Thefore, for safety, make sure the fallback codepaths only expand
the arguments once.

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