vndk-def: Fix test_elfdump.py

Before this commit, test_elfdump.py was broken because prebuilt libs
were removed from the "prebuilts" directory.  This commit searches the
NDK toolchain specified by "NDK" environment variable.

Test: ./tests/test_elfdump.py
Change-Id: I368d5ce2fa208dd00608fe0793995144ca0c9856
3 files changed