Rollup merge of #40457 - frewsxcv:frewsxcv-macos, r=steveklabnik

Update usages of 'OSX' (and other old names) to 'macOS'.

As of last year with version 'Sierra', the Mac operating system is now
called 'macOS'.

src/bootstrap/lib.rs

@@ -846,7 +846,7 @@ impl Build {
                            .filter(|s| !s.starts_with("-O") && !s.starts_with("/O"))
                            .collect::<Vec<_>>();
 
-        // If we're compiling on OSX then we add a few unconditional flags
+        // If we're compiling on macOS then we add a few unconditional flags
         // indicating that we want libc++ (more filled out than libstdc++) and
         // we want to compile for 10.7. This way we can ensure that
         // LLVM/jemalloc/etc are all properly compiled.