Move the 32-bit compatible procfs types from freebsd32.h to <sys/procfs.h>

Move the 32-bit compatible procfs types from freebsd32.h to <sys/procfs.h>
and export them to userland.
- Define __HAVE_REG32 on platforms that define a reg32 structure and check
  for this in <sys/procfs.h> to control when to export prstatus32, etc.
- Add prstatus32_t and prpsinfo32_t typedefs for the 32-bit structures.
  libbfd looks for these types, and having them fixes 'gcore' in gdb of a
  32-bit process on a 64-bit platform.
- Use the structure definitions from <sys/procfs.h> in gcore's elf32 core
  dump code instead of duplicating the definitions.

Differential Revision:	https://reviews.freebsd.org/D2142
Reviewed by:	kib, nathanw (powerpc bits)
MFC after:	1 week