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 hello_world_test
      out/host/linux-x86/testcases/hello_world_test/x86_64/hello_world_test
      out/host/linux-x86/testcases/hello_world_test/x86/hello_world_test

Change-Id: I0ecb1955cd67295abe12fc19ab811fe046ac5f23