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When llvm-nm finds no symbols in an object file (.o file or embedded in
in .a file) it prints 'no symbols' to stderr. This produces a lot of
noise when running 'supersize archive' from command line.

This CL makes SuperSize capture stderr for calls to llvm-nm (and nm,
although it seems to be more quiet). The number of stderr lines (all
'no symbols' as far as we see), if non-0, is returned by
nm.RunNmOnIntermediates(), and printed by obj_analyzer._RunNm() as a
warning.

Change-Id: I2090f8762614d5fc3102f0ec94ef7e83fc305bfd
Reviewed-on: https://chromium-review.googlesource.com/c/1289989Reviewed-by: Samuel Huang <huangs@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601031}