Fixes to V8 fetch process on AIX

Disable download of prebuild clang binaries on AIX platform when fetching
v8. Affects AIX only.

Change-Id: Ib3ed02c2cb292e4815904cbe5fedfca19c347f81
Reviewed-on: https://chromium-review.googlesource.com/919187Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#51407}

DEPS

@@ -335,6 +335,8 @@ hooks = [
     # Note: On Win, this should run after win_toolchain, as it may use it.
     'name': 'clang',
     'pattern': '.',
+    # clang not supported on aix
+    'condition': 'host_os != "aix"',
     'action': ['python', 'v8/tools/clang/scripts/update.py'],
   },
   {