Darwin, libgcc : Adjust min version supported for the OS.

Darwin, libgcc : Adjust min version supported for the OS.

Tools from later versions of the OS deprecate or fail to support
earlier OS revisions.

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

	* config.host: Arrange to set min Darwin OS versions from
	the configured host version.
	* config/darwin10-unwind-find-enc-func.c: Do not use current
	headers, but declare the nexessary structures locally to the
	versions in use for Mac OSX 10.6.
	* config/t-darwin: Amend to handle configured min OS
	versions.
	* config/t-darwin-min-1: New.
	* config/t-darwin-min-5: New.
	* config/t-darwin-min-8: New.

(cherry picked from commit 20b8779ea9bd82b26eeb195b30f695168cd7ae1d)