[cmake] Fix Findzstd.cmake to handle OpenBSD shared libraries

Fix Findzstd CMake to handle shared libraries on OpenBSD correctly.
This userland does not use shared library symlinks without SOVERSION,
so the result of find_library() does not ever end with
zstd_SHARED_LIBRARY_SUFFIX.  To work around this, reverse the logic
to compare the result against zstd_STATIC_LIBRARY_SUFFIX and assume
shared library otherwise.

While at it, fix the conditions not to fall back to "result is static
library" path if it actually was recognized as a shared library but
zstd_shared target already existed.

Fixes #59056

Differential Revision: https://reviews.llvm.org/D138361