More consistent host library path in multilib build.

In 64-bit multilib host build, changed from
32-bit lib: out/host/<platform>/lib32
64-bit lib: out/host/<platform>/lib
to
32-bit lib: out/host/<platform>/lib
64-bit lib: out/host/<platform>/lib64
.
That way the host library path is consistent with the multilib target
build's. Also with this change prebuilt 32-bit libraries can be reused
in 64-bit host build as 2nd arch binaries. (With previous setup, they
can't be used because they have rpath ../lib in it while the 2nd arch
library path needs ../lib32.

Change-Id: I020199d0c7dd52cdc8dcb7d3a1d22cd6178672e1