ALSA: pcm_native - fix runtime->boundary calculation

The code in pcm_lib updating runtime->hw_ptr_interrupt expects
that runtime->boundary is divisible with runtime->period_size.
Thanks are going to Clemens Ladisch for the notice.

Fix the runtime->boundary calculation using buffer_size * period_size
as base and find a least common multiple for 32bit platforms when
the expression might overflow.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>