minijail: build and install constants.json

This is a syscall table used for compiling Minijail policy files to
BPF.  The compiler is available in the minijail-tools package.  The
file is generated by compiling and running a small program named
dump_constants.

When cross-compiling, we have to get the syscall table for the host
platform.  To do this, dump_constants is run under QEMU user emulation
for the appropriate platform.  Google takes the same approach in their
minijail packages for ChromiumOS[1].

[1]: https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/729bd4269a47870380f8dde0b162de355ce89dfe/chromeos-base/minijail/minijail-9999.ebuild#49