Make the tests possible

[farcaller]

All ioreg access is wrapped in VolatileCell, so it should be simple to provide ioreg mocks in tests. It's however not possible to build zinc with --test, as rust-core conflicts with std.

A way to fix that is to drop rust-core and use bare minimum from libstd, so that native builds without no_std are possible.

[bharrisau]

Can wait for rust-lang/rfcs#40 to get implemented. Otherwise I've managed to build on my rust/cortex project using the normal libstd (it is a little clunky building libstd, need to do things like ar cr libbacktrack.a and remove the crate_type for dylib).

[bharrisau]

Like this https://github.com/bharrisau/rs-cortex/blob/master/examples/blink/Makefile#L36

[farcaller]

I see you also got libcompiler-rt, libmorestack and liblibc in. I figured out that removing all those deps would be untrivial and hard to maintain. I might actually try to build all four libs, that sounds like a better option in long term.

[bharrisau]

Most of those are empty archives just to keep the linker happy when it includes -lbacktrace.

[farcaller]

Zinc is now using libstd.