libmd: strip local absolute symbols (to link with lld)

Old versions of gas produce an invalid section index. That is ignored by
old versions of ld, but prevents a link with lld.

Submitted by: Rafael Ávila de Espíndola (earlier version)
Reviewed by: allanjude
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6789