[libc++] Refactor the selection of string's ABI in __config

[ldionne]

Summary

This doesn't change the selection, but it expands the conditions to
add comments and make it clearer what's happening. It also removes a
-Wundef instance when we checked ARM_ARCH_7K >= 2 without checking
that it is defined in the first place.

[Mordante]

Thanks, this is quite more readable. LGTM!

libcxx/include/__config

 #  if !defined(__cpp_exceptions) || __cpp_exceptions < 199711L
 #    define _LIBCPP_HAS_NO_EXCEPTIONS
 #  endif
 
 #  define _LIBCPP_PREFERRED_ALIGNOF(_Tp) __alignof(_Tp)
 
 #  if defined(_LIBCPP_COMPILER_CLANG_BASED)
 
-#    if defined(__APPLE__) && !defined(__i386__) && !defined(__x86_64__) && (!defined(__arm__) || __ARM_ARCH_7K__ >= 2)
-#      define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
-#    endif
+#    if defined(__APPLE__)
+#      if defined(__i386__) || defined(__x86_64__)
+// use old string layout on x86_64 and i386
+#      elif defined(__arm__)
+// use old string layout on arm (which does not include aarch64/arm64), except on watch ABIs
+#        if defined(__ARM_ARCH_7K__) && __ARM_ARCH_7K__ >= 2
+#          define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
+#        endif
+#      else
+#        define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
+#      endif
+#    endif
 
 // Objective-C++ features (opt-in)
 #    if __has_feature(objc_arc)
 #      define _LIBCPP_HAS_OBJC_ARC
 #    endif
 
 #    if __has_feature(objc_arc_weak)
 #      define _LIBCPP_HAS_OBJC_ARC_WEAK