[lib/Fuzzer] Add SHA1 implementation from public domain.

Summary:
This adds a SHA1 implementation taken from public domain code.
The change is trivial, but as it involves third-party code I'd like
a second pair of eyes before commit.

LibFuzzer can not use SHA1 from openssl because openssl may not be available
and because we may be fuzzing openssl itself.
Using sha1sum via a pipe is too slow.

Test Plan: n/a

Reviewers: chandlerc

Reviewed By: chandlerc

Subscribers: majnemer, llvm-commits

Differential Revision: http://reviews.llvm.org/D9733

llvm-svn: 237400