libgcc's DWARF unwinder (used for all Android targets except armv7) uses
dl_iterate_phdr, so we need to link against libdl to make the link
succeed. The driver ordinarily injects libdl for you, but we prevent the
driver's default library additions so we have to add it ourselves.
Ideally the libgcc linker script in the NDK would be adjusted to add the
libdl link, but until we have an NDK with that change, work around it in
the libc++ and libc++abi build systems.
Note that LIBCXXABI_HAS_DL_LIB was defined but previously unused, so I'm
just repurposing it. libunwind already links against libdl so no changes
are needed there.