Fix remaining using nuing = aliases

[stephentoub]

There are still a few places where we're explicitly defining nint/nuint as aliases for int/long/uint/ulong rather than using the new C# 9 types:

d:\repos\runtime\src\libraries\System.Private.CoreLib\src\System\Text\Latin1Utility.cs(13):using nint = System.Int64;
d:\repos\runtime\src\libraries\System.Private.CoreLib\src\System\Text\Latin1Utility.cs(14):using nuint = System.UInt64;
d:\repos\runtime\src\libraries\System.Private.CoreLib\src\System\Text\Latin1Utility.cs(16):using nint = System.Int32;
d:\repos\runtime\src\libraries\System.Private.CoreLib\src\System\Text\Latin1Utility.cs(17):using nuint = System.UInt32;
d:\repos\runtime\src\libraries\System.Private.CoreLib\src\System\Text\Unicode\Utf16Utility.Validation.cs(18):using nuint_t = System.UInt64;
d:\repos\runtime\src\libraries\System.Private.CoreLib\src\System\Text\Unicode\Utf16Utility.Validation.cs(20):using nuint_t = System.UInt32;
d:\repos\runtime\src\libraries\System.Private.CoreLib\src\System\Text\Unicode\Utf16Utility.Validation.cs(23):using nuint_t = System.UInt64;

cc: @GrabYourPitchforks

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[jkotas]

Fixing Utf16Utility.Validation.cs needs #36160.

[GrabYourPitchforks]

Vector<nuint_t> is only used in one place in that file. We could use if (IntPtr.Size == 8) { /* ... */ } else { /* ... */ } if we really needed to. It's not that much code duplication.

[tannergooding]

This was resolved a bit back (same time as #36160 was resolved)