archbuild: Add systemd-swap to create swapfile

roles/archbuild/files/systemd-swap.conf

+zswap[enabled]=1
+zswap[compressor]=lz4
+zswap[max_pool_percent]=20
+zswap[zpool]=z3fold
+
+swapf[size]=${sys[ram_size]}K
+swapf[path]=/var/swap

roles/archbuild/tasks/main.yml

 ---
 
 - name: install archbuild
-  pacman: name=devtools,zsh,git,subversion,mercurial,bzr,xdelta3 state=present
+  pacman: name=systemd-swap,devtools,zsh,git,subversion,mercurial,bzr,xdelta3 state=present
 
 - name: install archbuild scripts
   copy: src={{ item }} dest=/usr/local/bin/{{ item }} owner=root group=root mode=0755
@@ -30,9 +30,13 @@
     - mkpkg@.timer
     - mkpkg@.service
 
+- name: install systemd-swap config
+  copy: src=systemd-swap.conf dest=/etc/systemd-swap.conf owner=root group=root mode=0644
+
 - name: start and enable archbuild mounts
   service: name={{ item }} enabled=yes state=started
   with_items:
+    - systemd-swap.service
     - var-lib-archbuild.mount
     - var-lib-archbuilddest.mount