Request for review/comments: 32-bit compat for non-x86 architectures

[Nathan Whitehorn]

The patch at http://people.freebsd.org/~nwhitehorn/compat_freebsd32.diff 
(pre-generated freebsd32 syscalls stuff is included, which will be done 
in two steps on commit) provides groundwork for supporting 32-bit 
compatibility for 64-bit MIPS and PowerPC systems. It has been tested on 
amd64 and powerpc64, and compile-tested on ia64. There are two main 
parts to the patch:

1) COMPAT_IA32 is renamed COMPAT_FREEBSD32, in analogy to 
COMPAT_LINUX32, etc. This requires updating kernel configurations, but 
is less painful than filling machine-independent bits of the kernel with 
#if defined(COMPAT_IA32) || defined(COMPAT_PPC32) || 
defined(COMPAT_MIPS32) || ..., and is no less descriptive than the old name.

2) Modifications to the freebsd32 compat layer to support big-endian 
architectures.

I would appreciate any comments, bugs, or test results on ia64.
-Nathan