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