feat: Add proper support for IBM i

Python 3.9 on IBM i now properly returns "os400" for sys.platform
instead of claiming to be AIX as it did previously. While the IBM i PASE
environment is compatible with AIX, it is a subset and has numerous
differences which makes it beneficial to distinguish, however this means
that it now needs explicit support here.

addon.gypi

@@ -103,6 +103,11 @@
           '-Wl,-bimport:<(node_exp_file)'
         ],
       }],
+      [ 'OS=="os400"', {
+        'ldflags': [
+          '-Wl,-bimport:<(node_exp_file)'
+        ],
+      }],
       [ 'OS=="zos"', {
         'cflags': [
           '-q64',