[Darwin] The need for FDE symbols is dependent on linker used, not OS rev.

[Darwin] The need for FDE symbols is dependent on linker used, not OS rev.

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