This prevents a compiler warning from breaking ppc64le build.

Change-Id: I6752109bd02c6d078e656f89327093f8fb13a125
Reviewed-on: https://boringssl-review.googlesource.com/12363
Reviewed-by: Adam Langley <agl@google.com>

This change contains a C implementation of SHA-1 for POWER using
AltiVec. It is almost as fast as the scalar-only assembly implementation
for POWER/POWERPC family in OpenSSL but it is easier to maintain and it
allows error checking with tools like ASAN.

This is tested only for ppc64le. It may nor may not work for other
platforms in the POWER/POWERPC familiy.

Before:

SHA-1 @ 16 bytes: ~30 MB/s
SHA-1 @ 8K: ~140 MB/s

After:

SHA-1 @ 16 bytes: ~70 MB/s
SHA-1 @ 8K: ~480 MB/s

Change-Id: I790352e86d9c0cc4e1e57d11c5a0aa5b0780ca6b
Reviewed-on: https://boringssl-review.googlesource.com/12203
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>