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This relands commit 748ac91e.

Includes workaround for LTO bug exposed by orderfile generation:
add -Wl,-u,__cyg_profile_func_enter_bare to ldflags. A fix for the
bug is under review: https://reviews.llvm.org/D50017

Original change's description:
> Reland "build: Enable ThinLTO in official Android builds."
>
> This relands commit 1ba5d611.
>
> Fixes chromeos build issue by checking whether we have a secondary
> toolchain before comparing it with the current toolchain.
>
> Original change's description:
> > build: Enable ThinLTO in official Android builds.
> >
> > With this change we start building Chromium with ThinLTO when targeting
> > Android. In 64-bit Monochrome builds, the 32-bit binary is also built
> > with ThinLTO. This change is expected to reduce code size significantly
> > (in local measurements it reduced the size of MonochromePublic.apk by 2.3MB)
> > and is a prerequisite for enabling control flow integrity in official
> > builds.
> >
> > Note that although this change enables ThinLTO, it does not enable
> > the cross-TU optimizations normally associated with LTO. Enabling
> > cross-TU optimizations is a separate project.
> >
> > On my local machine (a Lenovo P920) libmonochrome.so link times increase
> > as follows:
> >
> >          before  after (first link)  after (incremental links)
> > 32-bit    18s          140s                     50s
> > 64-bit    13s          107s                     41s
> >
> > All known blockers have now been fixed. This CL is likely to uncover
> > unknown blockers.
> >
> > Bug: 469376
> > Change-Id: I8981e17abd50bc5ca00440e0d74dda878c911749
> > Reviewed-on: https://chromium-review.googlesource.com/1150870
> > Commit-Queue: Peter Collingbourne <pcc@chromium.org>
> > Reviewed-by: Nico Weber <thakis@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#578195}
>
> Bug: 469376, 868426
> Change-Id: Ie46ba0db0ac68130bc600415777de5c813b28ebd
> Reviewed-on: https://chromium-review.googlesource.com/1153612
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Commit-Queue: Peter Collingbourne <pcc@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#578854}

Bug: 469376, 867878
Change-Id: Iae4cc698b5955df5ae1970c71646fa05fca61109
Reviewed-on: https://chromium-review.googlesource.com/1155977Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579308}