libmd: stop exporting _block symbols

These are needed across compilation units so we can keep the _libmd_
prefixing bits (though I suspect we're not likely to collide), but we
don't need to be exporting the unprefixed versions of these; it's an
implementation detail.

PR: 280784 (exp-run)
Reviewed by: delphij, fuz
Differential Revision: https://reviews.freebsd.org/D34498