The original implementation hard-coded the target to x86-32.

The new implementation mirrors pnacl-llc by probing the architecture it is running on.

Continues the plumbing work in https://codereview.chromium.org/1698523003/ , toward the goal of enabling sandboxed translator tests for x86-64, particularly on the spec bots.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4077
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1696123003 .