tests/tcg: add float_madds test to multiarch

tests/tcg: add float_madds test to multiarch

This is a generic floating point multiply and accumulate test for
single precision floating point values. I've split of the common float
functions into a helper library so additional tests can use the same
common code.

As I don't have references for all architectures I've allowed some
flexibility for tests to pass without reference files. They can be
added as we get collect them.

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