Build System: Fix rpath error if native executable under testcase folder.

error while loading shared libraries: libc++.so:
cannot open shared object file: No such file or directory

BUG: N/A
Test: make -j16 liblatinime_host_unittests
      out/host/linux-x86/testcases/liblatinime_host_unittests/x86_64/liblatinime_host_unittests

Change-Id: I97738f728c29ab904fc979ff9b08a8d5bd1328cd