All mips64 has compat_netbsd32

branches: 1.2.8;
According to mrg@ we need to include aarch64 in the list of those
that need to support COMPAT_NETBSD32

branches: 1.1.4;
don't assume _LP64 == supports COMPAT_NETBSD32.

this is not true for alpha, ia64 and arm32 ports, and the first two
were not building because of it, and the latter would be missing
the oabi support (likely not a big deal, but still wrong.)

add a makefile fragment that tells you if it is supported and include
it where needed to define COMPAT_NETBSD32 when building the normal
kernel (ie, modules & rump.)


fixes alpha build, probably fixes ia64 build.


XXX: still leaves some netbsd32 code in rf_netbsdkintf.c, that should
be moved into some hooks, but first the configuration setup
needs to be moved into a common function the netbsd32 code can
call into, vs living in the switch case itself.