Darwin, D: Fix bootstrap when target does not support -Bstatic/dynamic.

Darwin, D: Fix bootstrap when target does not support -Bstatic/dynamic.

This fixes a bootstrap fail because saw_static_libcxx was unused for
targets without support for -Bstatic/dynamic.

The fix applied pushes the -static-libstdc++ back onto the command
line, which allows a target to substitute a static version of the
c++ standard library using specs.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
gcc/d/ChangeLog:

	* d-spec.cc (lang_specific_driver): Push the -static-libstdc++
	option back onto the command line for targets without support
	for -Bstatic/dynamic.