[PATCH] Darwin, testsuite: -multiply_defined is obsolete

[FX Coudert]

With Xcode 15, gcc.dg/ssp-2.c fails due to a warning: -multiply_defined is 
obsolete

The patches ignores the warning when present.
OK to push?

FX

0001-Darwin-testsuite-multiply_defined-is-obsolete.patch
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