libphobos, testsuite: Add prune clauses for two Darwin cases.

libphobos, testsuite: Add prune clauses for two Darwin cases.

Depending on the permutation of CPU, OS version and shared/non-
shared library inclusion, we get can get two warnings from the
external tools (ld64, dsymutil) which are not actually GCC issues
but relate to the external tools.  These are alrrady pruned in
the main testsuite, this adds them to the library.

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

	* testsuite/lib/libphobos.exp: Prune warnings from external
	tool bugs.

(cherry picked from commit 532fae74c3b1197023a765272892e12e123f0a56)