Cross-target static-image helper execution for cross-compilation

The static-init helper currently runs on the build host and produces static images using the host's libn00b layout. ncc rejects builds where the requested target ABI differs from the build-host target ABI.

Future work: support cross-ABI compilation (e.g., 32-bit host → 64-bit target, or different platform target). Two plausible directions:
1. A cross-ABI-aware helper invocation that consults the target's libn00b layout descriptors rather than the host's.
2. Explicit ABI separation at build time: ship per-target prebuilt helpers in libn00b, and have ncc pick the right one based on the configured target.

Documented in WP-007 DF-008 and reiterated in WP-011 deferrals as a watch item. No runtime user has hit this yet because all current consumers cross-compile only between same-ABI targets.

Start by reading the static-init helper and ncc's target-ABI handling, then review the libn00b layout descriptors and the WP-007 DF-008 and WP-011 deferrals. Compare the two proposed cross-ABI approaches and define which target-specific helper or descriptor is selected. Done means cross-ABI builds no longer reject a differing host and target ABI.

Written by the indexing model from the issue text.