build: Add 'DL' silent rule.

* Makefile.am (AM_V_DL, AM_V_DL_, AM_V_DL_0): New variables.
* gnu-system.am (gnu/packages/bootstrap/x86_64-linux/guile-2.0.9.tar.xz)
(gnu/packages/bootstrap/i686-linux/guile-2.0.9.tar.xz)
(gnu/packages/bootstrap/armhf-linux/guile-2.0.11.tar.xz)
(gnu/packages/bootstrap/mips64el-linux/guile-2.0.9.tar.xz): Use $(AM_V_DL).
* build-aux/download.scm: Print a line break between target and origin.