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This is because GCC also uses the -B search path at link time to find
"as", and our bundled "as" can only target x86. See also the discussion at
https://codereview.chromium.org/1084133002/

(I also tried adding this condition around line 858, but this won't work
because of scoping issues ("clang" is not defined here)).

R=thakis@chromium.org,fdegans@chromium.org
BUG=469376

Review URL: https://codereview.chromium.org/1096553002

Cr-Commit-Position: refs/heads/master@{#325545}