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'.