Make it possible for code to query the dynamic linker's default search path.

We're not going to have init(1) set LD_LIBRARY_PATH globally on 64-bit.
This patch makes it possible for libnativehelper to set LD_LIBRARY_PATH
in each Java VM (to support System.loadLibrary) without also hard-coding
the default search path there.

Change-Id: If13961fae976e06dd80d5ef522f31e8b7eb01154