[master] Update dependencies from dotnet/corefx (dotnet/coreclr#26965)

[master] Update dependencies from dotnet/corefx (dotnet/coreclr#26965)

* Update dependencies from https://github.com/dotnet/corefx build 20190930.12

- Microsoft.NETCore.Platforms - 5.0.0-alpha1.19480.12
- Microsoft.Private.CoreFx.NETCoreApp - 5.0.0-alpha1.19480.12

* Update dependencies from https://github.com/dotnet/corefx build 20191001.13

- Microsoft.NETCore.Platforms - 5.0.0-alpha1.19501.13
- Microsoft.Private.CoreFx.NETCoreApp - 5.0.0-alpha1.19501.13

* Update dependencies from https://github.com/dotnet/corefx build 20191002.6

- Microsoft.NETCore.Platforms - 5.0.0-alpha1.19502.6
- Microsoft.Private.CoreFx.NETCoreApp - 5.0.0-alpha1.19502.6

* Update dependencies from https://github.com/dotnet/corefx build 20191003.5

- Microsoft.NETCore.Platforms - 5.0.0-alpha1.19503.5
- Microsoft.Private.CoreFx.NETCoreApp - 5.0.0-alpha1.19503.5

* Disable newly added Utf8SpanTests

* Update dependencies from https://github.com/dotnet/corefx build 20191004.9

- Microsoft.NETCore.Platforms - 5.0.0-alpha1.19504.9
- Microsoft.Private.CoreFx.NETCoreApp - 5.0.0-alpha1.19504.9

* Update dependencies from https://github.com/dotnet/corefx build 20191005.5

- Microsoft.NETCore.Platforms - 5.0.0-alpha1.19505.5
- Microsoft.Private.CoreFx.NETCoreApp - 5.0.0-alpha1.19505.5

Commit migrated from https://github.com/dotnet/coreclr/commit/93a3714f509ed71460d2a315ec6280dc3d54df8d