libgcc: hide CIE and FDE data for DWARF architecture extensions behind a handler.

This patch provides a new handler MD_ARCH_FRAME_STATE_T to hide an
architecture-specific structure containing CIE and FDE data related
to DWARF architecture extensions.

Hiding the architecture-specific attributes behind a handler has the
following benefits:
1. isolating those data from the generic ones in _Unwind_FrameState
2. avoiding casts to custom types.
3. preserving typing information when debugging with GDB, and so
   facilitating their printing.

This approach required to add a new header md-unwind-def.h included at
the top of libgcc/unwind-dw2.h, and redirecting to the corresponding
architecture header via a symbolic link.

An obvious drawback is the increase in complexity with macros, and
headers. It also caused a split of architecture definitions between
md-unwind-def.h (types definitions used in unwind-dw2.h) and
md-unwind.h (local types definitions and handlers implementations).
The naming of md-unwind.h with .h extension is a bit misleading as
the file is only included in the middle of unwind-dw2.c. Changing
this naming would require modification of others backends, which I
prefered to abstain from. Overall the benefits are worth the added
complexity from my perspective.

libgcc/ChangeLog:

	* Makefile.in: New target for symbolic link to md-unwind-def.h
	* config.host: New parameter md_unwind_def_header. Set it to
	aarch64/aarch64-unwind-def.h for AArch64 targets, or no-unwind.h
	by default.
	* config/aarch64/aarch64-unwind.h
	(aarch64_pointer_auth_key): Move to aarch64-unwind-def.h
	(aarch64_cie_aug_handler): Update.
	(aarch64_arch_extension_frame_init): Update.
	(aarch64_demangle_return_addr): Update.
	* configure.ac: New substitute variable md_unwind_def_header.
	* unwind-dw2.h (defined): MD_ARCH_FRAME_STATE_T.
	* config/aarch64/aarch64-unwind-def.h: New file.
	* configure: Regenerate.
	* config/no-unwind.h: Updated comment

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