Repair assembly/C offset synchronization for runtime_sh4_minimal.S

[drpaneas]

Summary

The runtime still relies on hardcoded .equ offsets in runtime/runtime_sh4_minimal.S, but the current gen-offsets.c / asm-offsets.h pipeline is incomplete and make check-offsets does not validate the assembly constants actually in use.

Why

• Assembly context-switch code depends on struct field offsets matching C layouts exactly.
• The current generated runtime/asm-offsets.h is only a placeholder with include guards and no offset definitions.
• make check-offsets validates that placeholder output, not the hardcoded .equ constants consumed by the assembly.
• This creates silent-corruption risk: a C struct layout change can make the assembly read or write the wrong fields.

Evidence

• runtime/runtime_sh4_minimal.S still defines local .equ constants such as G_CONTEXT.
• runtime/asm-offsets.h currently contains only include guards and no offset values.
• Makefile explicitly notes that check-offsets does not validate the hardcoded .equ values.
• runtime/gen-offsets.c still exists and is intended to describe the C-side layout being checked.

Direction

• Make one source of truth for assembly-visible offsets.
• Either generate a real include file consumed by assembly, or add a direct build-time/runtime verification step against the .equ constants actually used.
• Ensure make check-offsets fails when assembly constants drift from the C layout.
• Document the supported workflow for changing G / context-related structs once the synchronization path is real.