[foozzie] Fix infering architecture

Broke in:
https://chromium-review.googlesource.com/c/v8/v8/+/725706

TBR=sergiyb@chromium.org
NOTRY=true

Bug: v8:6917
Change-Id: Ieb91c06a6bc1447acbe68c9ce881478c9a4a6149
Reviewed-on: https://chromium-review.googlesource.com/727800Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48719}

tools/foozzie/v8_foozzie.py

@@ -127,7 +127,7 @@ def infer_arch(d8):
   executable.
   """
   with open(os.path.join(os.path.dirname(d8), 'v8_build_config.json')) as f:
-    arch = json.load(f)['v8_current_cpu']
+    arch = json.load(f)['v8_target_cpu']
   return 'ia32' if arch == 'x86' else arch