Move the ASMC_DEBUG code out of asmc.c into a file compiled only when the
option is set, gated in files.amd64 the way acpi_debug gates the ACPICA
debugger sources. The module Makefile writes the define into opt_asmc.h so
every translation unit sees it; the old CFLAGS.asmc.c form covered one.
No functional change.
MFC After: 1 Week
Approved-by: ngie (co-mentor)