libstdc++, testsuite: Add a prune expression for external tool bug.

Depending on the permutation of CPU, OS version and shared/non-
shared library inclusion, we get can get warnings from the external
tools (ld64, dsymutil) which are not actually libstdc++ issues but
relate to the external tools themselves.  This is already pruned
in the main testsuite, this adds it to the library.

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

libstdc++-v3/ChangeLog:

	* testsuite/lib/prune.exp: Prune dsymutil (ld64) warning.