[wasm coreclr] fix bigint

[radekdoulik]

Make coreclr work with -sWASM_BIGINT=1

It fails with

RuntimeError: memory access out of bounds
    at corerun.wasm.emscripten_memset_bulkmem (wasm://wasm/corerun.wasm-19cb4c66:wasm-function[33089]:0xc5072c)
    at corerun.wasm.__memset (wasm://wasm/corerun.wasm-19cb4c66:wasm-function[33088]:0xc5071d)
    at corerun.wasm.MethodTableBuilder::AllocateWorkingSlotTables() (wasm://wasm/corerun.wasm-19cb4c66:wasm-function[17922]:0x642822)
    at corerun.wasm.MethodTableBuilder::BuildMethodTableThrowing(LoaderAllocator*, Module*, Module*, unsigned int, BuildingInterfaceInfo_t*, MethodTableBuilder::bmtLayoutInfo const*, MethodTable*, MethodTableBuilder::bmtGenericsInfo const*, SigPointer, unsigned short) (wasm://wasm/corerun.wasm-19cb4c66:wasm-function[17883]:0x639222)
    at corerun.wasm.ClassLoader::CreateTypeHandleForTypeDefThrowing(Module*, unsigned int, Instantiation, AllocMemTracker*) (wasm://wasm/corerun.wasm-19cb4c66:wasm-function[18261]:0x675bdf)
    at corerun.wasm.ClassLoader::CreateTypeHandleForTypeKey(TypeKey const*, AllocMemTracker*) (wasm://wasm/corerun.wasm-19cb4c66:wasm-function[7579]:0x2a1dd7)
    at corerun.wasm.ClassLoader::DoIncrementalLoad(TypeKey const*, TypeHandle, ClassLoadLevel) (wasm://wasm/corerun.wasm-19cb4c66:wasm-function[7578]:0x2a1a5a)
    at corerun.wasm.ClassLoader::LoadTypeHandleForTypeKey_Body(TypeKey const*, TypeHandle, ClassLoadLevel) (wasm://wasm/corerun.wasm-19cb4c66:wasm-function[7520]:0x29bdef)
    at corerun.wasm.ClassLoader::LoadTypeHandleForTypeKey(TypeKey const*, TypeHandle, ClassLoadLevel, InstantiationContext const*) (wasm://wasm/corerun.wasm-19cb4c66:wasm-function[7516]:0x29ab12)
    at corerun.wasm.ClassLoader::LoadTypeDefThrowing(Module*, unsigned int, ClassLoader::NotFoundAction, ClassLoader::PermitUninstantiatedFlag, unsigned int, ClassLoadLevel, Instantiation*) (wasm://wasm/corerun.wasm-19cb4c66:wasm-function[7463]:0x295568)

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[jkotas]

Context #119640 (comment)

This can be closed once we upgrade to EMSDK that enables this by default.