Changes to MI Linux emulation code necessary to run 32-bit Linux binaries

Changes to MI Linux emulation code necessary to run 32-bit Linux binaries
on AMD64, and the general case where the emulated platform has different
size pointers than we use natively:
- declare certain structure members as l_uintptr_t and use the new PTRIN
  and PTROUT macros to convert to and from native pointers.
- declare some structures __packed on amd64 when the layout would differ
  from that used on i386.
- include <machine/../linux32/linux.h> instead of <machine/../linux/linux.h>
  if compiling with COMPAT_LINUX32. This will need to be revisited before
  32-bit and 64-bit Linux emulation support can coexist in the same kernel.
- other small scattered changes.

This should be a no-op on i386 and Alpha.