For very old toolchains, the compiler generated extra symbols that mark the
start of each FDE.  We no longer need this (since xcode 3 era).

Since we have detection of the linker version, we can use that directly to
determine if support is needed.

2019-06-18  Iain Sandoe  <iain@sandoe.co.uk>

	* config/darwin.c (darwin_emit_unwind_label): New default to false.
	(darwin_override_options): Set darwin_emit_unwind_label as needed.

From-SVN: r272409