doc: update required compiler level for AIX

Compilers were updated for 10.X as per
discussion in: nodejs/build#925

PR-URL: #20153
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

Author: mhdawson

BUILDING.md

@@ -84,6 +84,9 @@ Depending on host platform, the selection of toolchains may vary.
 * GCC 4.9.4 or newer
 * Clang 3.4.2 or newer
 
+#### AIX
+* GCC 6.3 or newer
+
 #### Windows
 
 * Visual Studio 2017 or the Build Tools thereof