[wasm coreclr] Simplify gcenv for wasm, reuse NativeAOT's implementation

[radekdoulik]

Context: #116383 (comment)

Try to reuse NativeAOT's implementation.
dotnet/runtimelab#1510

We should also think about https://github.com/dotnet/runtimelab/blob/d2267785df3e3f9f590705e413cd9ae72f86ffb4/src/coreclr/gc/wasm/gcenv.wasm.cpp#L65-L66

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