require certain make_variables to be explicit

context: https://lore.kernel.org/llvm/CAKwvOdmXvC-E_9PSOZVQDDx_UCSadxhF4d9c7qpJPmeFLY_JEA@mail.gmail.com/

Right now, I think CROSS_COMPILE, HOSTCC, and CC (maybe CROSS_COMPILE_COMPAT and ARCH) make_variables are implied by other flags.

From the above lore links, a few of those are now implied by other flags (mostly LLVM=1), and we'd like to be able to test those flags being unspecified for a kernel build / invocation of make.

This means we'll need to update our tuxsuite files to add back any variables we need to be explicit now, but I suspect there wont be many, or that will only be the case for older branches.