Darwin: Handle linker '-demangle' option.

Most of the Darwin linkers in use support this option which we will
now pass by default (matching the Xcode clang impl.)>

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

gcc/ChangeLog:

	* config.in: Regenerate.
	* config/darwin.h (DARWIN_LD_DEMANGLE): New.
	(LINK_COMMAND_SPEC_A): Add demangle handling.
	* configure: Regenerate.
	* configure.ac: Detect linker support for '-demangle'.