x86: x86 user_regset math_emu

This converts the ptrace/signal accessors for i387 math_emu
state to the user_regset interface style, and calls these
from the old interfaces.

It also cleans up math_emulate's ptrace check to be a
single-step check, which is what it really wants.

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