Darwin: Move checking of the 'shared' driver spec.

Darwin: Move checking of the 'shared' driver spec.

This avoids a bunch of irrelevant diagnostics if the user passes '-shared' to
gnatmake.  Currently, we push '-dynamiclib' back onto the command line (since
that is the Darwin spelling of 'shared') but this is not handled by gnat1,
leading to a diagnostic for every character after the '-d'.

'-shared' has no effect on gnatmake (it needs to be passed to gnatbind).

This moves the handling of '-shared' to leaf specs so that we do not need to
push 'dynamiclib' onto the command line.

gcc/ChangeLog:

	* config/darwin.h:
	(SUBTARGET_DRIVER_SELF_SPECS): Move handling of 'shared' into the same
	specs as 'dynamiclib'. (STARTFILE_SPEC): Handle 'shared'.

(cherry picked from commit 2e36eedb244badaaf2a70388071115c851b8db9b)