testsuite, Darwin: Remove an unnecessary flags addition.

The addition of the multiply_defined suppress flag has been handled for some
considerable time now in the Darwin specs; remove it from the testsuite libs.
Avoid duplicates in the specs.

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

gcc/ChangeLog:

	* config/darwin.h: Avoid duplicate multiply_defined specs on
	earlier Darwin versions with shared libgcc.

libstdc++-v3/ChangeLog:

	* testsuite/lib/libstdc++.exp: Remove additional flag handled
	by Darwin specs.

gcc/testsuite/ChangeLog:

	* lib/g++.exp: Remove additional flag handled by Darwin specs.
	* lib/obj-c++.exp: Likewise.

(cherry picked from commit 3c776fdf)