Fix architecture detection in layout_tests Linux port.
Fix architecture detection in layout_tests Linux port.

Correctly determine architecture of position-independent executables. Those show
up as shared objects in the output of the file utility.

Also, fix a comment.

BUG=397119
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/741043002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@185740 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs