set target for darwin binary compat in skarnet.org packages

skalibs:
execline:
s6-dns:
s6-networking:
s6-portable-utils:
s6-rc:
s6:

The above software uses the target triplet from `cc -dumpmachine` as a
binary compatibility check. However, on darwin, the output includes the
darwin version number, which leads to build failures against a binary
skalibs package built a different version of darwin than the current
system.

Explicitly setting target ensures code can be compiled against a skalibs
binary built on a different version of darwin.

See http://www.skarnet.org/cgi-bin/archive.cgi?1:mss:623:heiodchokfjdkonfhdph