tools: use macOS as operating system name

As of 2016, the operating system is macOS. Remove use of "macosx".

PR-URL: #20579
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

BUILDING.md

@@ -133,15 +133,15 @@ More Developer Tools...`. This step will install `clang`, `clang++`, and
 If the path to your build directory contains a space, the build will likely
 fail.
 
-After building, setting up [firewall rules](tools/macosx-firewall.sh) can avoid
+After building, setting up [firewall rules](tools/macos-firewall.sh) can avoid
 popups asking to accept incoming network connections when running tests.
 
 Running the following script on macOS will add the firewall rules for the
 executable `node` in the `out` directory and the symbolic `node` link in the
 project's root directory.
 
 ```console
-$ sudo ./tools/macosx-firewall.sh
+$ sudo ./tools/macos-firewall.sh
 ```
 
 On FreeBSD and OpenBSD, you may also need: