AMDGCN: Disable subdirectory configuration for unsupported GAS and LD

The `amdgcn' target is not supported by GAS or LD, so disable building
in the respective subdirectories, removing configuration errors such as:

This target is no longer supported in gas
make[1]: *** [Makefile:5473: configure-gas] Error 1

and:

*** ld does not support target amdgcn-unknown-none
*** see ld/configure.tgt for supported targets
make[1]: *** [Makefile:6968: configure-ld] Error 1

in builds where no explicit `--disable-gas' and `--disable-ld' options
have been used with the top-level `configure' script.  Users must not
have to disable features selected by default to get a working
configuration.

	/
	* configure.ac <amdgcn-*-*> (noconfigdirs): Add `ld' and `gas'.
	* configure: Regenerate.