doc: add arm64 macOS as experimental

PR-URL: #36189
Refs: nodejs/build#2474
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ash Cripps <acripps@redhat.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Pranshu Srivastava <rexagod@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

Author: richardlau

BUILDING.md

@@ -113,6 +113,7 @@ platforms. This is true regardless of entries in the table below.
 | Windows          | x64, x86         | Windows Server 2012 (not R2)    | Experimental |                                   |
 | Windows          | arm64            | >= Windows 10                   | Tier 2 (compiling) / Experimental (running) |    |
 | macOS            | x64              | >= 10.13                        | Tier 1       |                                   |
+| macOS            | arm64            | >= 11                           | Experimental |                                   |
 | SmartOS          | x64              | >= 18                           | Tier 2       |                                   |
 | AIX              | ppc64be >=power7 | >= 7.2 TL02                     | Tier 2       |                                   |
 | FreeBSD          | x64              | >= 11                           | Experimental | Downgraded as of Node.js 12  <sup>[7](#fn7)</sup>     |