These implementations are incorrect (eg f2d and d2f don't handle special
values like 0.0) and proper versions can be provided by libgcc (or
equivalent depending on the toolchain).
libgcc is now linked with the stmhal port so that library will provide
these functions from now on.
acoshf, asinhf, atanhf were added from musl. mathsincos.c was
split up into its original, separate files (from newlibe-nano-2).
tan was added.
All of the important missing float functions are now implemented,
and pyboard now passes tests/float/math_fun.py (finally!).
Top-level lib directory is for standard C libraries that we want to
provide our own versions of (for efficiency and stand-alone reasons).
It currently has libm in it for math functions.
Also add atanf and atan2f, which addresses issue #837.
