[libc] Fix installed 'math.h' header paths

Summary:
Recent changes added an include path in the float128 type that used the
internal `libc` path to find the macro. This doesn't work once it's
installed because we need to search from the root of the install dir.
Move the include to the top-level math.h so we can find it.

Author: jhuber6

libc/include/CMakeLists.txt

@@ -99,6 +99,7 @@ add_gen_header(
   DEPENDS
     .llvm_libc_common_h
     .llvm-libc-macros.math_macros
+    .llvm-libc-macros.float_macros
     .llvm-libc-types.double_t
     .llvm-libc-types.float_t
     .llvm-libc-types.float128

libc/include/llvm-libc-types/float128.h

@@ -9,8 +9,6 @@
 #ifndef __LLVM_LIBC_TYPES_FLOAT128_H__
 #define __LLVM_LIBC_TYPES_FLOAT128_H__
 
-#include <include/llvm-libc-macros/float-macros.h> // LDBL_MANT_DIG
-
 // Currently, C23 `_Float128` type is only defined as a built-in type in GCC 7
 // or later, and only for C.  For C++, or for clang, `__float128` is defined
 // instead, and only on x86-64 targets.

libc/include/math.h.def

@@ -11,9 +11,9 @@
 
 #include <__llvm-libc-common.h>
 #include <llvm-libc-macros/math-macros.h>
+#include <llvm-libc-macros/float-macros.h>
 #include <llvm-libc-types/float128.h>
 
-
 %%public_api()
 
 #endif // LLVM_LIBC_MATH_H