--Remove NULL-checked dmamap macros. These are a minor pessimization on every
configuration besides x86+bounce, as no other configuration allows NULL maps.
arm64 originally allowed NULL maps, which seems to have been the motivaation
behind adding arm-specific memory barriers to bus_dma.h, but this was
removed in r299463.
--Remove sparc64 exception for dmamap function declarations, and implement
the previous macros as functions in bus_machdep.c
--Declare (optional) interface to MD implementation in bus_dma_internal.h