gcn: Update --with-arch= for newer archs

Replace hard-coded list of supported devices by directly checking
config/gcn/gcn-devices.def.

gcc/ChangeLog:

	* config.gcc (--with-{arch,tune}): Use .def file to validate gcn
	processor names.
	* doc/install.texi (amdgcn*-*-*): Update list of devices supported
	by --with-arch/--with-tune.

(cherry picked from commit 61a6430cf663e3c980c2ee966f094fea7d99f8e7)