config.mk: #define MAC as well

MAC is also almost universally a default; every GENERIC includes it, and
it's std.armv[67]. mips is again the oddball here with it only being
included in ERL/OCTEON1.

The only module currently working around this one is mac_veriexec, but it
looks like nothing it builds actually uses the MAC definition. Downstream
consumers enabling MAC in mips using mac_veriexec may be advised to do
something differently here in config.mk.