c++, driver: Fix -static-libstdc++ for targets without Bstatic/dynamic.

c++, driver: Fix -static-libstdc++ for targets without Bstatic/dynamic.

The current implementation for swapping between the static and shared c++
runtimes relies on the static linker supporting Bstatic/dynamic which is
not available for every target (Darwin's linker does not support this).

Specs substitution (%s) is an alternative solution for this (which is what
Darwin uses for Fortran, D and Objective-C).  However, specs substitution
requires that the '-static-libstdc++' be preserved in the driver's command
line.  The patch here arranges for this to be done when the configuration
determines that linker support for Bstatic/dynamic is missing.

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

	* g++spec.cc (lang_specific_driver): Preserve -static-libstdc++ in
	the driver command line for targets without -Bstatic/dynamic support
	in their static linker.