Add -ldl to libc++ tests when sanitizers are used.

Clang 3.6 no longer links the sanitizer runtime library dependancies when
-nodefaultlibs is used. This patch manually links in a missing dependancy.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@220463 91177308-0d34-0410-b5e6-96231b3b80d8