Take advantage of nint and nuint in Utf8String code

[eerhardt]

When we get a compiler with the new nint/nuint feature we should change our current ifdef pattern in the Utf8String code:

runtime/src/libraries/System.Private.CoreLib/src/System/Text/Unicode/Utf8Utility.Transcoding.cs

Lines 14 to 20 in 490f09e

	#if TARGET_64BIT
	using nint = System.Int64;
	using nuint = System.UInt64;
	#else // TARGET_64BIT
	using nint = System.Int32;
	using nuint = System.UInt32;
	#endif // TARGET_64BIT

And just use the new feature.

For the OOB Utf8String package, we chose to use Int64 and UInt64 for nint and nuint, because we can't compile twice for each bitness.

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[jkotas]

This is in no way specific to Utf8String.

[eerhardt]

I can make it more general to cover the whole CoreLib. I was just logging this in order to put in a tracking issue for this comment on my PR.

It directly affects Utf8String because we can only build one IL assembly for the OOB package, and so we had to pick what nint means in that build.

[jkotas]

Fixed by #36159. One remaining place is left that would need #36160.