It makes more sense to keep constants.json in the minijail package,
because that's where the tool that consumes it,
compile_seccomp_policy, lives. By having it in this package, we can
set it as the default location for compile_seccomp_policy, which means
it shouldn't ever even need to be specified on the command
line (although it still can be). And we can hook into the
cross-compilation machinery to get it to automatically use the
constants for the right architecture.
I've also changed from generating constants.json by running a test
program in qemu-user to generating it from LLVM IR, which will save a
huge QEMU build dependency.
eb38d95b8a