share/mk: Pass -znoexecstack to ld.bfd when linking libraries and programs.

lld assumes -znoexecstack by default whereas ld.bfd still defaults to
-zexecstack in the absence of .note.GNU-stack annotations. Adding the
flags centrally avoids having to patch various libraries in the tree
as one-offs (e.g. OpenSSL 3 is the current thing generating new
warnings with ld.bfd).

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D41120