VROUND, FSTCW and STMXCSR all have to perform the same conversion from
x86 rounding modes to softfloat constants.  Since the ISA is consistent
on the meaning of the two-bit rounding modes, extract the common code
into a wrapper for set_float_rounding_mode.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>