[libcxx] Document new symbols __u64toa and __u32toa on Darwin

[ldionne]

Summary

This is the counterpart for https://reviews.llvm.org/D50130 and
https://reviews.llvm.org/D52391 on Darwin.

lib/abi/CHANGELOG.TXT

* r338479 - Elementary string conversions for integral types
 
   x86_64-linux-gnu
   ----------------
   Symbol added: _ZNSt3__16__itoa8__u64toaEmPc
   Symbol added: _ZNSt3__16__itoa8__u32toaEjPc
 
   x86_64-apple-darwin16.0
   -----------------------
+  Symbol added: __ZNSt3__16__itoa8__u64toaEyPc
+  Symbol added: __ZNSt3__16__itoa8__u32toaEjPc
+
 
 * r333467 - Fix embarrasing typo in uncaught_exceptions.
 
   This bug caused __uncaught_exception to be ODR used instead of
   __uncaught_exceptions. This change is non-ABI breaking because the symbols
   for std::uncaught_exception and std::uncaught_exceptions haven't changed,
   and because users shouldn't be depending directly on libc++ exporting
   __uncaught_exception/__uncaught_exceptions.

lib/abi/x86_64-apple-darwin.v1.abilist

 {'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__15stoldERKNS_12basic_stringIwNS_11char_traitsIwEENS_9allocatorIwEEEEPm'}
 {'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__15stollERKNS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEEPmi'}
 {'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__15stollERKNS_12basic_stringIwNS_11char_traitsIwEENS_9allocatorIwEEEEPmi'}
 {'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__15stoulERKNS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEEPmi'}
 {'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__15stoulERKNS_12basic_stringIwNS_11char_traitsIwEENS_9allocatorIwEEEEPmi'}
 {'type': 'OBJECT', 'is_defined': True, 'name': '__ZNSt3__15wcerrE', 'size': 0}
 {'type': 'OBJECT', 'is_defined': True, 'name': '__ZNSt3__15wclogE', 'size': 0}
 {'type': 'OBJECT', 'is_defined': True, 'name': '__ZNSt3__15wcoutE', 'size': 0}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__16__itoa8__u32toaEjPc'}
+{'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__16__itoa8__u64toaEyPc'}
 {'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__16__sortIRNS_6__lessIaaEEPaEEvT0_S5_T_'}
 {'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__16__sortIRNS_6__lessIccEEPcEEvT0_S5_T_'}
 {'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__16__sortIRNS_6__lessIddEEPdEEvT0_S5_T_'}
 {'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__16__sortIRNS_6__lessIeeEEPeEEvT0_S5_T_'}
 {'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__16__sortIRNS_6__lessIffEEPfEEvT0_S5_T_'}
 {'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__16__sortIRNS_6__lessIhhEEPhEEvT0_S5_T_'}
 {'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__16__sortIRNS_6__lessIiiEEPiEEvT0_S5_T_'}
 {'type': 'FUNC', 'is_defined': True, 'name': '__ZNSt3__16__sortIRNS_6__lessIjjEEPjEEvT0_S5_T_'}