[libc++] Fix abs and div overload issue for compilers on AIX

Summary:
AIX system's stdlib.h provide different overload of abs and div
depending on compiler versions.

For example, std::div(long, long) and std::abs(long) are not available
from OS's stdlib.h when building with clang, but they are available
when building with xlclang compiler.

Therefore, we need to provide those extra overloads in libc++'s stdlib.h
when OS's stdlib.h does not.

Differential Revision: https://reviews.llvm.org/D99767

Author: jasonliudev

libcxx/include/cmath

@@ -335,9 +335,7 @@ using ::isunordered;
 using ::float_t;
 using ::double_t;
 
-#ifndef _AIX
 using ::abs;
-#endif
 
 using ::acos;
 using ::acosf;

libcxx/include/stdlib.h

@@ -103,7 +103,7 @@ extern "C++" {
 #endif
 
 // MSVCRT already has the correct prototype in <stdlib.h> if __cplusplus is defined
-#if !defined(_LIBCPP_MSVCRT) && !defined(__sun__) && !defined(_AIX)
+#if !defined(_LIBCPP_MSVCRT) && !defined(__sun__)
 inline _LIBCPP_INLINE_VISIBILITY long abs(long __x) _NOEXCEPT {
   return __builtin_labs(__x);
 }
@@ -112,9 +112,9 @@ inline _LIBCPP_INLINE_VISIBILITY long long abs(long long __x) _NOEXCEPT {
   return __builtin_llabs(__x);
 }
 #endif // _LIBCPP_HAS_NO_LONG_LONG
-#endif // !defined(_LIBCPP_MSVCRT) && !defined(__sun__) && !defined(_AIX)
+#endif // !defined(_LIBCPP_MSVCRT) && !defined(__sun__)
 
-#if !(defined(_AIX) || defined(__sun__))
+#if !defined(__sun__)
 inline _LIBCPP_INLINE_VISIBILITY float abs(float __lcpp_x) _NOEXCEPT {
   return __builtin_fabsf(__lcpp_x); // Use builtins to prevent needing math.h
 }
@@ -127,7 +127,7 @@ inline _LIBCPP_INLINE_VISIBILITY long double
 abs(long double __lcpp_x) _NOEXCEPT {
   return __builtin_fabsl(__lcpp_x);
 }
-#endif // !(defined(_AIX) || defined(__sun__))
+#endif // !defined(__sun__)
 
 // div
 
@@ -138,7 +138,7 @@ abs(long double __lcpp_x) _NOEXCEPT {
 #endif
 
 // MSVCRT already has the correct prototype in <stdlib.h> if __cplusplus is defined
-#if !defined(_LIBCPP_MSVCRT) && !defined(__sun__) && !defined(_AIX)
+#if !defined(_LIBCPP_MSVCRT) && !defined(__sun__)
 inline _LIBCPP_INLINE_VISIBILITY ldiv_t div(long __x, long __y) _NOEXCEPT {
   return ::ldiv(__x, __y);
 }
@@ -148,7 +148,7 @@ inline _LIBCPP_INLINE_VISIBILITY lldiv_t div(long long __x,
   return ::lldiv(__x, __y);
 }
 #endif // _LIBCPP_HAS_NO_LONG_LONG
-#endif // _LIBCPP_MSVCRT / __sun__ / _AIX
+#endif // _LIBCPP_MSVCRT / __sun__
 } // extern "C++"
 #endif  // __cplusplus