Add a BHYVE_GDB_SUPPORT make variable that can be set by per-arch
makefiles. When set, BHYVE_GDB is defined and can be used as a
preprocessor predicate. Use it to guard gdb stub calls in MI code.

The arm64 bhyve port currently does not have a functional gdb stub, but
that's not critical to landing the port, so this mechanism slightly
reduces the friction of adding support for a new platform.