fix: provide host function stubs for ABI generation

[r-near]

Summary

• Adds stub implementations of NEAR host functions so the native dylib linker step succeeds during ABI generation
• Fixes the case where contracts using the sys crate for direct host API access (e.g. no_mangle extern functions) cause link failures with undefined symbols like _attached_deposit, _storage_read, etc.
• Uses RTLD_GLOBAL stubs + macOS linker flags approach (WIP — to be reworked with cargo rustc approach)

Closes #317

Test plan

• Verify ABI generation works for contracts using direct sys crate host API access
• Test against house-of-stake-contracts repro case from the issue
• Confirm no regressions on standard contract ABI generation