signal/all: remove return value from restore_sigcontext

make most implementations of restore_sigcontext void and
remove checking it's return value from functions calling
restore_sigcontext.

The exception is the X86 version of the function that is
too different from others to deal in this way, and arm
version, to keep possibility of erroring out from failed
valid_user_regs.

v3: keep arm valid_user_regs for filling in near future.

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
