[libcxx][modules] Add a submodule for math.h

[bruno]

Summary

Adding a module for this header is one step towards self hosting clang with submodules local visibility on darwin.

[bruno]

This isn't needed anymore.

include/module.modulemap

 /*
     export_macros FP_FAST_FMA, FP_FAST_FMAF, FP_FAST_FMAL, FP_ILOGBO, FP_ILOGBNAN,
                   FP_INFINITE, FP_NAN, FP_NORMAL, FP_SUBNORMAL, FP_ZERO,
                   HUGE_VAL, HUGE_VALF, HUGE_VALL, INFINITY, NAN,
                   MATH_ERRNO, MATH_ERREXCEPT, math_errhandling
 */
     export *
   }
+  module math_h {
+    header "math.h"
+    export *
+  }
   module codecvt {
     header "codecvt"
     export *
   }
   module complex {
     header "complex"
     export *
   }