Add global variable HOST_LIBRARY_PATH.

With multilib host build, the build system installs host
shared libraries to different directories depending on a
library's bitness:
- HOST_OUT_SHARED_LIBRARIES points to the library path of 64-bit;
- 2ND_HOST_OUT_SHARED_LIBRARIES points to the library path of 32-bit;
- If you don't care the bitness of the libraries and just want whatever
  version the librareies are built by default, use HOST_LIBRARY_PATH.

Bug:13751317
Change-Id:Id4c818941dc4ea35d795767c76f698529bd6aebb