Darwin: Rework handling for unwinder code in libgcc_s and specs [PR80556].

This addresses a long-standing problem where a work-around for an unwinder
issue (also a regression) regresses other functionality.  The patch replaces
several work-arounds with a fix for PR80556 and a work-around for PR88590.

* The fix for PR80556 requires a bump to the SO name for libgcc_s, since we
need to remove the unwinder symbols from it.  This would trigger PR88590
hence the work-around for that.

* We weaken the symbols for emulated TLS support so that it is possible
for a DSO linked with static-libgcc to interoperate with a DSO linked with
libgcc_s.  Likewise main exes.

* We remove all the gcc-4.2.1 era stubs machinery and workarounds.

* libgcc is always now linked ahead of libc, which avoids fails where the
libc (libSystem) builtins implementations are not up to date.

* The unwinder now always comes from the system
 - for Darwin9 from /usr/lib/libgcc_s.1.dylib
 - for Darwin10 from /usr/lib/libSystem.dylib
 - for Darwin11+ from /usr/lib/system/libunwind.dylib.

 We still insert a shim on Darwin10 to fix an omitted unwind function, but
 the underlying unwinder remains the system one.

* The work-around for PR88590 has two parts (1) we always link libgcc from
its convenience lib on affected system versions (avoiding the need to find
the DSO path); (2) we add and export the emutls functions from DSOs - this
makes a relatively small (20k) addition to a DSO.  These can be backed out
when a proper fix for PR88590 is committed.

For distributions that wish to install a libgcc_s.1.dylib to satisfy linkage
from exes that linked against the stubs can use a reexported libgcc_s.1.1
(since that contains all the symbols that were previously exported via the
 stubs).

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

gcc/ChangeLog:

	PR target/80556
	* config/darwin-driver.c (darwin_driver_init): Handle exported
	symbols and symbol lists (suppress automatic export of the TLS
	symbols).
	* config/darwin.c (darwin_rename_builtins): Remove workaround.
	* config/darwin.h (LINK_GCC_C_SEQUENCE_SPEC): Likewise.
	(REAL_LIBGCC_SPEC): Handle revised library uses.
	* config/darwin.opt (nodefaultexport): New.
	* config/i386/darwin.h (PR80556_WORKAROUND): Remove.
	* config/i386/darwin32-biarch.h (PR80556_WORKAROUND): Likewise.
	* config/i386/darwin64-biarch.h (PR80556_WORKAROUND): Likewise.

libgcc/ChangeLog:

	* config.host: Add weak emutls crt to the extra_parts.
	* config/i386/darwin-lib.h (DECLARE_LIBRARY_RENAMES): Remove
	workaround.
	* config/libgcc-libsystem.ver: Add exclude list for the system-
	provided unwinder.
	* config/t-slibgcc-darwin: Bump SO version, remove stubs code.
	* config/i386/libgcc-darwin.10.4.ver: Removed.
	* config/i386/libgcc-darwin.10.5.ver: Removed.
	* config/rs6000/libgcc-darwin.10.4.ver: Removed.
	* config/rs6000/libgcc-darwin.10.5.ver: Removed.
	* config/t-darwin-noeh: New file.

gcc/testsuite/ChangeLog:

	* gcc.dg/torture/fp-int-convert-timode-3.c: Remove XFAIL.
	* gcc.dg/torture/fp-int-convert-timode-4.c: Likewise.

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