[mono] Fix r8->i conversion on amd64.

[vargaz]

Fixes #64386.

[lambdageek]

@vargaz should we backport this to 6.0?

[vargaz]

We should

[am11]

@vargaz, is this failure related https://helix.dot.net/api/2019-06-17/jobs/a5963b9d-9f2a-495e-8451-7569cebe9bd4/workitems/System.Runtime.InteropServices.Tests/console:

    System.Runtime.InteropServices.Tests.NFloatTests.NFloatToIntPtr(value: 0) [FAIL]
      Assert.Equal() Failure
      Expected: 0
      Actual:   4393751543808
      Stack Trace:
        /_/src/libraries/System.Runtime.InteropServices/tests/System.Runtime.InteropServices.UnitTests/System/Runtime/InteropServices/NFloatTests.cs(483,0): at System.Runtime.InteropServices.Tests.NFloatTests.NFloatToIntPtr(Single value)
        /_/src/mono/System.Private.CoreLib/src/System/Reflection/RuntimeMethodInfo.Mono.cs(386,0): at System.Reflection.RuntimeMethodInfo.InvokeWorker(Object obj, BindingFlags invokeAttr, Span`1 parameters)
    System.Runtime.InteropServices.Tests.NFloatTests.NFloatToIntPtr(value: 4567) [FAIL]

[lambdageek]

/backport to release/6.0

[github-actions]

Started backporting to release/6.0: https://github.com/dotnet/runtime/actions/runs/1843163912