re PR middle-end/17982 (stop calling assemble_external before final assembly output time)

	PR 17982
	* varasm.c (pending_assemble_externals): New static.
	(assemble_external_real): Meat of assemble_external split out
	to this new function.
	(process_pending_assemble_externals): New function.
	(assemble_external): Use gcc_assert.  If flag_unit_at_a_time
	is true and the basic test passes, merely cons the decl onto
	the pending list to be handled later.
	* tree.h: Declare process_pending_assemble_externals.
	* cgraphunit.c (cgraph_optimize): Call it.

	* config/h8300/h8300.h: Do not define ASM_OUTPUT_EXTERNAL.

From-SVN: r91914