Add libdl.a from prebuilts/ndk.

We aren't quite ready to fully move to the new static libraries, but
we need libdl.a for using libc++ in a static executable (the unwinder
uses dladdr).

Test: ndk/checkbuild.py && ndk/run_tests.py
Bug: None
Change-Id: Ibc2ba4b27cc1fc1fb229719ea3c3c37cb8819fd3