add support for arch-specific dependencies

[cjdcordeiro]

Atm, we support slice contents that are application specific, however, there's no way to specify an essential which is only required for a specific architecture.

Example: libnode108 only needs libatomic1 for riscv64.

[linostar]

Format suggestion for essential architecture-supported entry:
<slice>:<arch>

For example:
libatomic1_libs:riscv64

In case of a slice required for more than one architecture, a separate line is to be used for each, following the same format above.

[endersonmaia]

Maybe we could support some kind of negative rule, suppose that you want to exclude some arch.

For ex.: criu is not available for riscv64 so one would use criu:!riscv64 or even a list like criu:!{riscv64, amd64} (not a real scenario for criu here).

[dulmandakh]

I'm having same issue with canonical/chisel-releases#253. Chisel needs architecture support in dependencies