lib/libc/tests/string: add extended unit tests for strcmp()

This changeset add a new set of tests that comprehensively test strcmp() on
various alignments of the input. This made it easy to smoke out many
exciting new bugs in the new SSE strcmp() implementation from D41971.

MFC after: 1 week
Reviewed by: ngie
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41970