Add ability to force compile as 64 bit.

When trying to build a single 64 bit and 32 bit image, there are
a few executables that are prefer32. When set, this 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: I84f6be7fe0fa3a9ea86ecc4dd785aaf1825f9925