Install 64-bit libraries to /system/lib64.

/system/lib always contains 32-bit libraries, and /system/lib64 (if
present) always contains 64-bit libraries.
Move things around a little bit, so TARGET_ARCH can be used to define
the build paths.

Bug: 11654773
Change-Id: I2edd91e162c7a20d7719d7bae15e5fa6c2a5b498