Hide struct pcb definition by #ifdef __amd64__ braces.  If cc -m32
compilation results in inclusion of the header, a confict arises due
to savefpu being union for i386, but used as struct in the pcb
definition.  The 32bit code should not need amd64 variant of the
struct pcb anyway.

For struct region_descriptor, use __uint64_t instead of unsigned long,
as the base type for bit-fields.  Unsigned long cannot have width 64
for -m32.

The changes allowed to use sys/sysctl.h for cc -m32.

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