[build] Switch random deopt fuzzer to x64

The new deopt stress is implemented for x64 only.

NOTRY=true
TBR=sergiyb@chromium.org

Bug: v8:6900
Change-Id: I542cb89de643c477875893f6b0b73ced44108b9e
Reviewed-on: https://chromium-review.googlesource.com/708259
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48414}

infra/mb/mb_config.pyl

@@ -104,7 +104,7 @@
       'V8 Linux - predictable': 'gn_release_x86_predictable',
       'V8 Linux - full debug': 'gn_full_debug_x86',
       'V8 Linux - interpreted regexp': 'gn_release_x86_interpreted_regexp',
-      'V8 Random Deopt Fuzzer - debug': 'gn_debug_x86',
+      'V8 Random Deopt Fuzzer - debug': 'gn_debug_x64',
     },
     'client.v8.clusterfuzz': {
       'V8 Mac64 ASAN - release builder':