Add DDB to opt_ddb.h

[zbb]

Summary

Some DDB related files (i.e. armv6 debug_monitor.c) should not
be compiled-in when DDB is disabled but on the other hand stub
replacements are supposed to be called.
This patch allows for compilation time inclusion/exclusion of the above
mentioned code.

[zbb]

Ping?

[imp]

You break these files, which don't currently include opt_ddb.h, but
do have #ifdef DDB in them:
sys/arm/arm/pmap-v6.c
sys/arm/arm/pmap.c
sys/arm64/arm64/trap.c

And may break files that include these widely included files.

sys/sys/cpuset.h
sys/sys/interrupt.h
sys/sys/lockmgr.h
sys/sys/sx.h

Though it looks like the right answer for those is to simply remove the #ifdef DDB
block and always declare the function. The linker will throw an error if someone
is calling the function w/o properly including DDB in the kernel. I believe that's
already the case, but haven't audited it beyond looking at the first two files closely.
It's generally bad form to have #ifdef based on options in the .h files in the kernel
anyway, especially for widely included stuff.

[zbb]

No longer needed