softfloat: Fix float64_to_uint64_round_to_zero

The float64_to_uint64_round_to_zero routine is incorrect.

For example, the following test pattern:

    46697351FF4AEC29 / 0x1.97351ff4aec29p+103

currently produces 8000000000000000 instead of FFFFFFFFFFFFFFFF.

This patch re-implements the routine to temporarily force the
rounding mode and use the float64_to_uint64 routine.

This contribution can be licensed under either the softfloat-2a or -2b
license.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Message-id: 1387397961-4894-4-git-send-email-tommusta@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>