x86: clean up ptrace.h

Leave definition of pt_regs in its own section, move all kernel
code to section afterwards, unify prototype definitions, has some
conditional prototypes to make it clear what was only defined in
32 and 64 bit.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>