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PTRACE_GET_THREAD_AREA is not in ptrace.h in Ubuntu 16.04 when
building for Linux without a sysroot so get it from somewhere else,
in this case asm/ptrace-abi.h.

Follow up to https://chromium-review.googlesource.com/c/1479092

Bug: 934238,933418
Change-Id: I3e1ace31a6f5a4ab65bbe712e1516d7b5fa009a5
Reviewed-on: https://chromium-review.googlesource.com/c/1481316Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#634660}