Darwin: Use -platform_version when available [PR110624].

Later versions of the static linker support a more flexible flag to
describe the OS, OS version and SDK used to build the code.  This
replaces the functionality of '-mmacosx_version_min' (which is now
deprecated, leading to the diagnostic described in the PR).

We now use the platform_version flag when available which avoids the
diagnostic.

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

	PR target/110624

gcc/ChangeLog:

	* config/darwin.h (DARWIN_PLATFORM_ID): New.
	(LINK_COMMAND_A): Use DARWIN_PLATFORM_ID to pass OS, OS version
	and SDK data to the static linker.