Instead of duplicating this existing logic across the source file. This
will make it easier to add additional linux targets (e.g. linux-musl) in
the future.
* guix/build/syscalls.scm (readdir*, write-socket-address!)
(read-socket-address): Use linux? constant.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>
Change-Id: I833c1d1630dcb8319584de1ea918cb22696f0058