libstdc++: Sync the atomic_link_flags implementation with GCC.

For Darwin, in order to allow uninstalled testing, we need to provide
a '-B' option pointing to each path containing an uninstalled library
that we are using (these get appended to the embedded runpaths).

This updates the version of the atomic_link_flags proc in the libstdc++
testsuite to do the same as the one in the GCC testsuite.

libstdc++-v3/ChangeLog:

	* testsuite/lib/dg-options.exp (atomic_link_flags): Emit a -B
	option for the path to the uninstalled libatomic.

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