Change the operating system name from OSX to MacOS everywhere

Summary:
- Changed the name in all build artifacts
- Everywhere in C++

Reviewers: #swift_pilot_client, kbasan

Reviewed By: #swift_pilot_client, kbasan

Subscribers: jenkins

Maniphest Tasks: T205

Differential Revision: https://dev.swift-project.org/D69

src/blackgui/stylesheetutility.cpp

@@ -482,7 +482,7 @@ namespace BlackGui
         {
             specific = fn + ".win" + qss;
         }
-        else if (CBuildConfig::isRunningOnMacOSXPlatform())
+        else if (CBuildConfig::isRunningOnMacOSPlatform())
         {
             specific = fn + ".mac" + qss;
         }