linux-user: fix x86_64 safe_syscall

Do what the comment says, test for signal_pending non-zero,
rather than the current code which tests for bit 0 non-zero.

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