x86: x86 user_regset general regs

This adds accessor functions in the user_regset style for
the general registers (struct user_regs_struct).

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>