gclient: Add aix to host_os list

R=mmoss@chromium.org, dpranke@chromium.org, phajdan.jr@chromium.org

Bug:
Change-Id: I0bfe0c38f3b1bfa5c8bd40747a0c3483d823cbdc
Reviewed-on: https://chromium-review.googlesource.com/660722Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>

gclient.py

@@ -1165,6 +1165,7 @@ _PLATFORM_MAPPING = {
   'darwin': 'mac',
   'linux2': 'linux',
   'win32': 'win',
+  'aix6': 'aix',
 }
 
 
@@ -1177,6 +1178,7 @@ class GClient(Dependency):
   solution or DEPS entry."""
 
   DEPS_OS_CHOICES = {
+    "aix6": "unix",
     "win32": "win",
     "win": "win",
     "cygwin": "win",