Fix ReadOnlySpanGetReferenceAndReadInteger on BigEndian

[EgorBo]

Addresses #85127 (comment)

[ghost]

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Issue Details

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Author:	EgorBo
Assignees:	-
Labels:	area-System.Memory
Milestone:	-

[EgorBo]

Wasm failures #85949, I assume it's not worth running extra-platforms pipeline for a trivial change.

[akoeplinger]

@EgorBo looks like the test still fails on s390x: https://dev.azure.com/dnceng-public/public/_build/results?buildId=273447&view=ms.vss-test-web.build-test-results-tab&runId=5405384&resultId=147326&paneView=debug

Assert.Equal() Failure
Expected: 28429436511322223
Actual:   7494100832430877696



Stack trace
   at System.SpanTests.MemoryMarshalTests.ReadOnlySpanGetReferenceAndReadInteger() in /_/src/libraries/System.Memory/tests/MemoryMarshal/GetReference.cs:line 113
   at System.Reflection.MethodInvoker.InterpretedInvoke(Object obj, IntPtr* args) in /_/src/mono/System.Private.CoreLib/src/System/Reflection/MethodInvoker.Mono.cs:line 30
   at System.Reflection.MethodInvoker.Invoke(Object obj, IntPtr* args, BindingFlags invokeAttr) in /_/src/libraries/System.Private.CoreLib/src/System/Reflection/MethodInvoker.cs:line 59