BUILD_HOST_64bit=1 to build host tools in 64-bit

By default we build tool in 32-bit.  This flag allow individual tool and its
dependencies to be built in 64-bit (eg. Emulator).  Fixes to resolve 64-bit
porting issues (in other git) will be submitted seperately.

Change-Id: I486cf7ddac727d3c374ed890857d497c3a69e598