Wasm NativeAOT: Add WASM_GLOBAL_INDEX_LEB relocation support + WasmNativeObjectWriter scaffold

[Copilot]

Wires in WASM_GLOBAL_INDEX_LEB relocation handling for the R2R Webcil path and lays the groundwork for a NativeAOT relocatable wasm object writer.

R2R (WasmObjectWriter.cs)

• Changed _definedSymbols[reloc.SymbolName] to TryGetValue — avoids KeyNotFoundException for non-ObjectNode symbols (e.g. WasmBaseGlobalSymbolNode) that are never added to _definedSymbols
• Added WASM_GLOBAL_INDEX_LEB case: maps __stack_pointer → 0, __memory_base → 1, __table_base → 2 (matching _defaultGlobalImports fixed positions)

WasmInstructions.cs

• Added Global.Get(ISymbolNode) / Global.Set(ISymbolNode) overloads using WasmLEBConstantReloc(..., WASM_GLOBAL_INDEX_LEB) — enables future code to reference base globals by symbol rather than hardcoded index

NativeAOT wasm writer scaffold

• WasmNativeObjectWriter.cs (new, Common/): skeleton for a relocatable wasm object writer (wasm magic/version header; all section/debug/unwind methods stubbed TODO-Wasm)
• ObjectWriter.cs: added static partial void TryCreateArchitectureSpecificWriter(...) hook using the existing partial method pattern
• ObjectWriter.Aot.cs: implements the hook to dispatch to WasmNativeObjectWriter for TargetArchitecture.Wasm32

Project files

• WasmBaseGlobalSymbolNode.cs added to both ILCompiler.ReadyToRun.csproj and ILCompiler.Compiler.csproj
• WasmAbiConstants.cs and WasmNativeObjectWriter.cs added to ILCompiler.Compiler.csproj