Restore default 'sorry' 'TARGET_ASM_CONSTRUCTOR', 'TARGET_ASM_DESTRUCTOR'

Restore default 'sorry' 'TARGET_ASM_CONSTRUCTOR', 'TARGET_ASM_DESTRUCTOR'

... that got lost in commit 7e0db0cdf01e9c885a29cb37415f5bc00d90c029
"STABS: remove -gstabs and -gxcoff functionality".

Previously, if a back end was not 'USE_COLLECT2', nor manually defined
'TARGET_ASM_CONSTRUCTOR', 'TARGET_ASM_DESTRUCTOR', or got pointed to the
respective 'default_[...]' functions due to 'CTORS_SECTION_ASM_OP',
'DTORS_SECTION_ASM_OP', or 'TARGET_ASM_NAMED_SECTION', it got pointed to
'default_stabs_asm_out_constructor', 'default_stabs_asm_out_destructor'.
These would emit 'sorry' for any global constructor/destructor they're
run into.

This is now gone, and thus in such a back end configuration case
'TARGET_ASM_CONSTRUCTOR', 'TARGET_ASM_DESTRUCTOR' don't get defined
anymore, and thus the subsequently following:

    #if !defined(TARGET_HAVE_CTORS_DTORS)
    # if defined(TARGET_ASM_CONSTRUCTOR) && defined(TARGET_ASM_DESTRUCTOR)
    # define TARGET_HAVE_CTORS_DTORS true
    # endif
    #endif

... doesn't define 'TARGET_HAVE_CTORS_DTORS' anymore, and thus per my
understanding, 'gcc/final.cc:rest_of_handle_final':

    if (DECL_STATIC_CONSTRUCTOR (current_function_decl)
        && targetm.have_ctors_dtors)
      targetm.asm_out.constructor (XEXP (DECL_RTL (current_function_decl), 0),
                                   decl_init_priority_lookup
                                     (current_function_decl));
    if (DECL_STATIC_DESTRUCTOR (current_function_decl)
        && targetm.have_ctors_dtors)
      targetm.asm_out.destructor (XEXP (DECL_RTL (current_function_decl), 0),
                                  decl_fini_priority_lookup
                                    (current_function_decl));

... simply does nothing anymore for a 'DECL_STATIC_CONSTRUCTOR',
'DECL_STATIC_DESTRUCTOR'.

This, effectively, means that GCC/nvptx now suddenly appears to "support"
global constructors/destructors, which means that a ton of test cases now
erroneously PASS that previously used to FAIL:

    sorry, unimplemented: global constructors not supported on this target

Of course, such support didn't magically happen due to
"STABS: remove -gstabs and -gxcoff functionality", so this is bad.  And,
corresponding execution testing then regularly FAILs (due to the global
constructor/destructor functions never being invoked), for example:

    [-UNSUPPORTED:-]{+PASS:+} gcc.dg/initpri1.c {+(test for excess errors)+}
    {+FAIL: gcc.dg/initpri1.c execution test+}

    [-UNSUPPORTED:-]{+PASS:+} g++.dg/special/conpr-1.C {+(test for excess errors)+}
    {+FAIL: g++.dg/special/conpr-1.C execution test+}

To restore the previous GCC/nvptx behavior, for traceability, this simply
restores the previous code, stripped down to the bare minimum.

	gcc/
	* Makefile.in (OBJS): Add 'dbxout.o'.
	* config/nvptx/nvptx.cc: '#include "dbxout.h"'.
	* dbxout.cc: New.
	* dbxout.h: Likewise.
	* target-def.h (TARGET_ASM_CONSTRUCTOR, TARGET_ASM_DESTRUCTOR):
	Default to 'default_stabs_asm_out_constructor',
	'default_stabs_asm_out_destructor'.