Add initial vbcc build path for Sevgi runtime

[cyberman]

This adds a first vbcc build path for the Sevgi runtime in Code/.

It introduces a separate Code/Makefile.vbcc and fixes a small number
of vbcc-exposed type/constant issues needed to complete a full runtime
build.

The runtime now compiles, assembles and links successfully with:

• vbcc
• vasmm68k_mot
• auto.lib + amiga.lib

Scope is intentionally limited to the runtime in Code/.
This does not claim vbcc support for the editor, tools, or the whole
repository yet.

[alpyre]

This is great.
Editor and tools do not require to be ported. Binaries will always be available on Aminet.
I don't have vbcc installed so I can not test.
Is there any other compiler warnings? I'd like a completely clean compilation as it is when compiled with gcc and SAS/C.

[cyberman]

Thanks, that's encouraging.

Yes, there are still vbcc warnings in Code/, but they are not all of the same kind.

So far, the meaningful ones fell into three categories:

• real type/constant issues (already fixed in this branch)
• a real LevelData initializer mismatch (found while checking warnings)
• legacy zero-length tail-array and bitfield warnings, which need to be separated into
  “intentional old-C layout pattern” vs “actual portability/design issue”

The next warning block I am currently checking is exactly that distinction, so I can report back with a smaller and better classified follow-up instead of mixing everything into one PR.