tests/fp/fp-test: add floating point tests

By leveraging berkeley's softfloat and testfloat.

With this we get decent coverage of softfloat.c:

$ ./fp-test -r even:	67.22% coverage
$ ./fp-test -r all:	73.11% coverage

Note that we do not yet test parts of softfloat.c that aren't
in the original softfloat library, namely:

- denormal inputs
- *_to_int16/uint16 conversions
- scalbn for fixed point
- muladd variants
- min/max
- exp2
- log2
- float*_compare (except float16_compare)

Signed-off-by: Emilio G. Cota <cota@braap.org>
[rth: Add the new modules to git_submodules.]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>