tests/fp: add fp-bench

tests/fp: add fp-bench

These microbenchmarks will allow us to measure the performance impact of
FP emulation optimizations. Note that we can measure both directly the impact
on the softfloat functions (with "-t soft"), or the impact on an
emulated workload (call with "-t host" and run under qemu user-mode).

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>