lib/libm: Add acosh, asinh, atanh, tan; get working with stmhal.

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!).

lib/libm/libm.h

@@ -19,6 +19,8 @@
 #include <stdint.h>
 #include <math.h>
 
+#define FLT_EVAL_METHOD 0
+
 #define FORCE_EVAL(x) do {                        \
 	if (sizeof(x) == sizeof(float)) {         \
 		volatile float __x;               \