dwarf2out: Emit BTF in dwarf2out_finish for BPF CO-RE usecase

dwarf2out: Emit BTF in dwarf2out_finish for BPF CO-RE usecase

DWARF generation is split between early and late phases when LTO is in effect.
This poses challenges for CTF/BTF generation especially if late debug info
generation is desirable, as turns out to be the case for BPF CO-RE.

The approach taken here in this patch is:

1. LTO is disabled for BPF CO-RE
The reason to disable LTO for BPF CO-RE is that if LTO is in effect, BPF CO-RE
relocations need to be generated in the LTO link phase _after_ the optimizations
are done. This means we need to devise way to combine early and late BTF. At
this time, in absence of linker support for BTF sections, it makes sense to
steer clear of LTO for BPF CO-RE and bypass the issue.

2. The BPF backend updates the write_symbols with BPF_WITH_CORE_DEBUG to convey
the case that BTF with CO-RE support needs to be generated.  This information
is used by the debug info emission routines to defer the emission of BTF/CO-RE
until dwarf2out_finish.

So, in other words,

dwarf2out_early_finish
  - Always emit CTF here.
  - if (BTF && !BTF_WITH_CORE), emit BTF now.

dwarf2out_finish
  - if (BTF_WITH_CORE) emit BTF now.

gcc/ChangeLog:

	* dwarf2ctf.c (ctf_debug_finalize): Make it static.
	(ctf_debug_early_finish): New definition.
	(ctf_debug_finish): Likewise.
	* dwarf2ctf.h (ctf_debug_finalize): Remove declaration.
	(ctf_debug_early_finish): New declaration.
	(ctf_debug_finish): Likewise.
	* dwarf2out.c (dwarf2out_finish): Invoke ctf_debug_finish.
	(dwarf2out_early_finish): Invoke ctf_debug_early_finish.