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.

Bug: 64503245
Test: ./tests/test_elfdump.py

Change-Id: Iee3076350c5d9a549deb4c08ac64453ef9dcd981
Merged-In: I368d5ce2fa208dd00608fe0793995144ca0c9856
(cherry picked from commit 3f9d07e2)