printm is specific to the FreeBSD dtrace port. I believe it's
effectively the same as tracemem(), though printm apparently predates
it. It stores the size of the buffer of traced data inline. Currently
it represents that size using a uintptr_t, which isn't really right and
poses challenges when porting to CHERI because
DTRACE_STORE(uintptr_t, ... requires the destination to be suitably
aligned, but this isn't necessary since we're just storing a size.
Convert to using a size_t. This should be a no-op since
sizeof(uintptr_t) == sizeof(size_t) on non-CHERI platforms (and I don't
see a reason to use printm() when tracemem() is available and is simpler
to use.)