All of the existing code was boilerplate from elsewhere,
and would crash the guest upon the first signal.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Stafford Horne <shorne@gmail.com>

---
v2:
  Add a comment to the new definition of target_pt_regs.
  Install the signal mask into the ucontext.
v3:
  Incorporate feedback from Laurent.