investigate --symbol-ordering-file

[nickdesaulniers]

https://reviews.llvm.org/D26130

I'm not sure how @GeorgiiR was creating these input files; there's a comment on an earlier version of the patch of just using readelf -ws for randomized order, but I recall CrOS folks discussing this at one point. I wonder if they already a file like this for clang?

[nickdesaulniers]

See also @glandium's https://glandium.org/blog/?p=1296.

[nickdesaulniers]

https://llvm.org/docs/BuildingADistribution.html#options-for-optimizing-llvm seems to sugguest there was some support for linker order files on OSX via dtrace. That documentation probably needs a revamp if it's possible to generate this on Linux.

https://reviews.llvm.org/D16134 seems to be the commit that did so; I wonder if we can do the equivalent with BPF?

[nickdesaulniers]

@gburgessiv found -Wl,--print-symbol-order=$_output_orderfile which would let us reuse an order file from a profile build, statically.