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/blackcore/application.cpp

@@ -532,8 +532,8 @@ namespace BlackCore
             QLatin1String(" Unix: ") %
             boolToYesNo(CBuildConfig::isRunningOnUnixPlatform()) %
             separator %
-            QLatin1String("MacOSX: ") %
-            boolToYesNo(CBuildConfig::isRunningOnMacOSXPlatform());
+            QLatin1String("MacOS: ") %
+            boolToYesNo(CBuildConfig::isRunningOnMacOSPlatform());
 
         return env;
     }