fpu: move inline helpers into a separate header

fpu: move inline helpers into a separate header

There are a bunch of users of the inline helpers who do not need
access to the entire softfloat API. Move those inline helpers into a
new header file which can be included without bringing in the rest of
the world.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>