Darwin: Fix empty g++ command lines [PR105599].

An empty g++ command line should produce a diagnostic that there are no
inputs.  The PR is that currently Darwin produces a dignostic about missing
link items instead - this is because (errnoeously), for this driver, we are
creating a link job for empty command lines.

The problem occurs in four stages:

 The g++ driver appends -shared-libgcc to the command line.

 The Darwin driver_init code in the backend does not see this (it sees an
 empty command line).

 When the back end driver code driver sees an empty command line, it does not
 add any supplementary flags (e.g. asm-macosx-version-min) - precisely to
 avoid anything being claimed as an input_file and therefore triggering a link
 line.

 Since we do not have a value for asm-macosx-version-min when processing the
 driver specs, we unconditionally inject 'multiply_defined suppress' which is
 used with shared libgcc (but only intended on very old Darwin).  This then
 causes the generation of a link job.

The solution, for the present, is to move version-specific link params to the
LINK_SPEC so that they are only processed when a link job has already been
decided.

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

	PR target/105599

gcc/ChangeLog:

	* config/darwin.h: Move versions-specific handling of multiply_defined
	from SUBTARGET_DRIVER_SELF_SPECS to LINK_SPEC.