libmd: Only define SHA256_Transform_c when using the ARM64 ifunc.

GCC 9 doesn't define a SHA256_Transform symbol when the stub just wraps
SHA256_Transform_c resulting in an undefined symbol for
_libmd_SHA256_Transform in libmd.so.

Discussed with: andrew, jrtc27
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D31945