Orderfile: add support for generating a 64 bit orderfile.

Note that the orderfile will be produced in orderfile.arm64.out,
which is currently not used when building (orderfile.arm.out is
used regardless of target_cpu). This makes sense as we use the
same orderfile on all architectures, but is potentially confusing.

Bug: 922460
Change-Id: I5f4b627529d3f9a4f20115a2609e512d83c4b88c
Reviewed-on: https://chromium-review.googlesource.com/c/1417833
Commit-Queue: Matthew Cary <mattcary@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Matthew Cary <mattcary@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625587}

• base/android/orderfile/BUILD.gn[diff]
• base/android/orderfile/orderfile_instrumentation.cc[diff]
• tools/cygprofile/orderfile_generator_backend.py[diff]