envkernel: Add --llvm flag to use LLVM

Compiling the kernel with LLVM may be desirable for several reasons, e.g. thin LTO, CFI or a couple of other clang-exclusive features in the kernel. This adds a --llvm flag to envkernel that will then ensure the kernel is built with an LLVM toolchain and sets LLVM=1 in the make invocation, as is the recommended way to compile the kernel with LLVM.