Genericizing sys/compat/netbsd32

[Matt Thomas <matt%3am-software.com@localhost>]

sys/compat/netbsd32 is great at running 32-bit NetBSD on a 64-bit kernel.
But with a little tweaking, it could do so much more.

For example, aarch64 will need multiple instances of compat_netbsd32
(one for arm32 eabi, one for arm32 oabi, and possibly one for aarch64
ilp32 unless it can use the arm32 eabi)

This requires being able to change the netbsd32_ that starts every function
to something unique.

Now if we are going that far, with a little more work we can separate out
the netbsd32 specific pieces and have a generic "netbsd" on netbsd compat
layer.  This could be used on ARM or some MIPS, or even PowerPC to run a
reverse endian userland (big endian user program on little endian kernel
for example).  Or improve the efficiency of running ARM OABI programs on
an EABI kernel (since much of the netbsd32 compatibility isn’t needed and
could be skipped).

I have started some effort towards this and have a set of diffs at
http://www.netbsd.org/~matt/netbsd32-diff.txt showing how syscalls could
be handled.  netbsd32_wait.c it the furthest along and being genericized.

I particularly like the NETBSDX_SYSCALL(foo) and 
NETBSDX_COMPAT_SYSCALL(n, foo) macros simplify things.