Make self linker outputs loadable shared objects for Ruby dlopen

[katsyoshi]

Goal:
Produce shared objects from gcc-generated object files that can be loaded at runtime (Ruby dlopen path).

Scope:

• Define and implement the minimum linker requirements for a loadable .so from gcc .o inputs.
• Prioritize runtime-load correctness over broad feature coverage.
• Cover the basic runtime path:
  1. dlopen succeeds
  2. expected symbol is resolvable (dlsym/equivalent)
  3. callable function returns expected value

Implementation plan (incremental):

• Start from a minimal -fPIC fixture (single exported function, no complex deps).
• Enumerate relocations emitted by gcc for that fixture and support required subset.
• Keep unsupported relocations as explicit linker errors (no silent output corruption).
• Add E2E test(s) in Ruby-side runtime loading path (Fiddle or equivalent wrapper).

Acceptance criteria:

• A self-linked .so from the minimal fixture is loadable and callable from Ruby runtime.
• Tests assert load + symbol resolution + function result.
• Failure mode for unsupported relocations is explicit and deterministic.

Out of scope (for this issue):

• Full relocation-family support across all gcc outputs.
• Property-based testing expansion (tracked separately).