libgcc, Darwin: Build a libgcc_s.1 for backwards compatibility.

libgcc, Darwin: Build a libgcc_s.1 for backwards compatibility.

In order to reslve a long-standing issue with inter-operation
with libSystem, we have bumped the SO name for libgcc_s.

Distributions might wish to install this new version into a
structure where exisiting code is already linked with the
compiler-local libgcc_s.1 (providing symbols exported by the
now-retired libgcc_ext.10.x shims).

The replacement libgcc_s.1 forwards the symbols from the new SO.
In order to support DYLD_LIBRARY_PATH on systems (where it works)
we forward the libSystem unwinder symbols from 10.7+ and a
compiler-local version of the libgcc unwinder on earlier.

For macOS 10.4 to 10.6 this is 'bug-compatible' with existing uses.
For 10.7+ the behaviour will now actually be correct.

This should be squashed with the initial libgcc changes for PR80556
in any backport (r12-5418-gd4943ce939d)

libgcc/ChangeLog:

	* config.host (*-*-darwin*): Add logic to build a shared
	unwinder library for Darwin8-10.
	* config/i386/t-darwin: Build legacy libgcc_s.1.
	* config/rs6000/t-darwin: Likewise.
	* config/t-darwin: Reorganise the EH fragments to place
	them for inclusion in a shared EH lib.
	* config/t-slibgcc-darwin: Build a legacy libgcc_s.1 and
	the supporting pieces (all FAT libs).
	* config/t-darwin-noeh: Removed.
	* config/darwin-unwind.ver: New file.
	* config/rs6000/t-darwin-ehs: New file.
	* config/t-darwin-ehs: New file.