Fix 64-bit host platform builds.

When performing a 64-bit host platform build, do not pick the 32-bit
SDL prebuilt libraries. This relies on the fact that BUILD_HOST_64bit
will be set in this case.

NOTE: This requires the X11 development headers/libraries to be
      installed on the machine, since SDL will be rebuilt directly
      from sources.

BUG=14489102

Change-Id: Ia36f73f8477059ea1a0d4c48c42fbe2d60595530
1 file changed