This makes powerpc64le-linux a supported architecture for Guix, but not for
Guix System.
* Makefile.am (SUPPORTED_SYSTEMS): Add an entry for powerpc64le-linux.
* etc/guix-install.sh (chk_sys_arch): Same.
* guix/packages.scm (%supported-systems): Same.
* m4/guix.m4 (GUIX_ASSERT_SUPPORTED_SYSTEM): Same.
* tests/guix-build.sh (all_systems): Same.