Better integrate default 'sorry' 'TARGET_ASM_CONSTRUCTOR', 'TARGET_ASM_DESTRUCTOR'

... after commit 4ee35c11
"Restore default 'sorry' 'TARGET_ASM_CONSTRUCTOR', 'TARGET_ASM_DESTRUCTOR'".
No functional change.

	gcc/
	* Makefile.in (OBJS): Remove 'dbxout.o'.
	* config/nvptx/nvptx.cc: Don't '#include "dbxout.h"'.
	* dbxout.cc: Remove.
	* dbxout.h: Likewise.
	* target-def.h (TARGET_ASM_CONSTRUCTOR, TARGET_ASM_DESTRUCTOR):
	Default to 'default_asm_out_constructor',
	'default_asm_out_destructor'.
	* targhooks.cc (default_asm_out_constructor)
	(default_asm_out_destructor): New.
	* targhooks.h (default_asm_out_constructor)
	(default_asm_out_destructor): Declare.