[CRT:MATH] Import nextafter, nextafterf and nexttowardf from musl

Note: On Windows, where a long double is the same as a double, nextafterl is the same as nextafter, nexttowardl is the same as nexttoward.
Also nexttoward is the same as nextafter.

sdk/lib/crt/math/math.cmake

@@ -14,6 +14,9 @@ list(APPEND LIBCNTPR_MATH_SOURCE
     math/div.c
     math/exp2f.c
     math/labs.c
+    math/nextafter.c
+    math/nextafterf.c
+    math/nexttowardf.c
     math/round.c
     math/roundf.c
     math/scalbn.c