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This CL introduces a fuzzer for the ELF disassemblers in Zucchini. I
have already uploaded some corpus files to the clusterfuzz-corpus
Google Storage bucket.

Achieves ~700 exec/s locally. This is on-par with the DEX and Win32
disassemblers as it requires a largish representative file to serve
as a test. (Recommendation is ~1000 exec/s).

Brings up coverage of ELF related code from 0-30% to 80-100%

I expect this will find quite a few crashes early on but should
stabilize within a couple of weeks; est. 1-2 bugfixes per day
based on how DEX and Win32 went. Mostly checked_casts and bounds
issues.

Bug: 1013641
Change-Id: I205135547cad2a95e59f99d7f040c13d72c45b59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1856624
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705245}