build: remove certctl requirement for host OpenSSL libs on macOS

Some platforms, like macOS, do not expose headers for the system's
libcrypto for public consumption. libcrypto is relatively heavy and
needs to know, e.g., the host system's endianness, so we scope the build
down to macOS where OpenSSL headers are known to not be present and we
can be reasonably certain that most of the systems today that would be
cross-building are little endian.

We still don't bother if building WITHOUT_OPENSSL since the end result
is expected to be used by OpenSSL, but perhaps we could revisit that
independently in case one, e.g., brings their own implementation.

Reported by: jrtc27
Reviewed by: jrtc27, ngie
Fixes: c340ef28fd38 ("certctl: Reimplement in C")
Differential Revision: https://reviews.freebsd.org/D51935