Force compile all targets as 64 bit.

When building this image, there are a few executables that are
prefer32. Setting this variable will force all of those targets to be
compiled as 64 bit.

Test: With this option, drmserver/mediaserver are built as 64 bit.
Test: Without this option, drmserver/mediaserver are built as 32 bit.
Change-Id: I520e381950ddb0b7bce52ca38a4fd27e4cf0683c