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

[jhb]

Summary

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).

[emaste]

share/mk/bsd.lib.mk

98

We might want to extend this, e.g. # LLD sensibly defaults to -znoexecstack, so do the same for BFD