configure, Darwin: Check ld64 support for -platform-version.

Newer versions of ld64 allow specifiying the OS target (e.g.
macos or ios) the version and the SDK version all in a single
command.  This checks the availability of the command for the
current toolchain.

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

gcc/ChangeLog:

	* config.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Test ld64 for -platform-version support.