amdgcn: deprecate Fiji device and multilib

LLVM wants to remove it, which breaks our build.  This patch means that
most users won't notice that change, when it comes, and those that do will
have chosen to enable Fiji explicitly.

I'm selecting gfx900 as the new default as that's the least likely for users
to want, which means most users will specify -march explicitly, which means
we'll be free to change the default again, when we need to, without breaking
anybody's makefiles.

gcc/ChangeLog:

	* config.gcc (amdgcn): Switch default to --with-arch=gfx900.
	Implement support for --with-multilib-list.
	* config/gcn/t-gcn-hsa: Likewise.
	* doc/install.texi: Likewise.
	* doc/invoke.texi: Mark Fiji deprecated.