testsuite, Darwin: Fix darwin-comm-1.c error messages for Darwin <= 10.

testsuite, Darwin: Fix darwin-comm-1.c error messages for Darwin <= 10.

When amending the allowed alignment size to accommodate the larger values
permitted by newer tools, we retained the object file limit of 2^15 for
Darwin versions <= 10, since that is what the native tools expect there.

This triggers a different diagnostic path with a distinct error message,
which is checked in the revised test here.

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

	* gcc.dg/darwin-comm-1.c: Check for the correct error message for
	Darwin <= 10.