I got problems when trying to compile libkvm.old, and I traced it
to /usr/src/sys/arch/mips/include/mips.h:

#if defined(MIPS1) && defined(MIPS3)
#error  Cannot yet upport both  "MIPS1" (r2000 family) and "MIP3" (r4000 family) in the same kernel.
#endif

#ifdef MIPS1
#include <mips/mips1_pte.h>
#endif

#ifdef MIPS3
#include <mips/mips3_pte.h>
#endif

The problem is that *neither* MIPS1 nor MIPS3 is defined.  I don't
know what the right way to resolve this is - while we cannot support
both MIPS1 and MIPS3 in the same kernel, can we do it in the same
libkvm? 

If not we'll need to define MIPS1 for pmax, or change the ifdef to be
#ifdef pmax or #ifdef _R3000.  (The libkvm.old Makefile is designed to
compile the kvm_${MACHINE_ARCH}.c = kvm_mips.c module, with the extra
flag -D${MACHINE} = -Dpmax, while gcc predefines _R3000 for us).

  -  Arne H. J.