cam: Make cam_debug macros atomic

The CAM_DEBUG* macros use multiple printfs to dump the data. This is
suboptimal when tracing things that produce even a moderate amount since
it gets intertwingled. I can't even turn on tracing with a 24-disk HBA
on boot without it getting messed up. Add helper routines to work around
clang's over-use of the stack: that way we only pay the stack penalty
when a trace hits.

Sponsored by: Netflix
Reviewed by: ken, mav
Differential Revision: https://reviews.freebsd.org/D42411