[build] Temporarily switch off custom libcxx on native arm debug

NOTRY=true
TBR=hablich@chromium.org

Bug: chromium:738814
Change-Id: I575a36e271fb70cbf9a9c09f0daf9056864487c5
Reviewed-on: https://chromium-review.googlesource.com/561143Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46435}

infra/mb/mb_config.pyl

@@ -346,7 +346,8 @@
 
     # GN debug configs for arm.
     'gn_debug_arm': [
-      'gn', 'debug_bot', 'arm', 'crosscompile', 'hard_float', 'swarming'],
+      'gn', 'debug_bot', 'arm', 'crosscompile', 'hard_float', 'swarming',
+      'no_custom_libcxx'],
 
     # GN release configs for arm.
     'gn_release_arm': [