Improve BMI2 MultiplyNoFlags APIs

[elbruno]

Improve BMI2 MultiplyNoFlags APIs
The current Bmi2.MultiplyNoFlags API follows the C/C++ intrinsic design that returns lower half of the result and stores the higher half in memory via a pointer of out argument. According to feedback dotnet/coreclr#18712 (comment), the API returning higher half is more useful for developers and simpler to implement and optimize for compiler writers. So, we suggest changing the API to:

public static unsafe uint MultiplyNoFlags(uint left, uint right);
public static unsafe uint MultiplyNoFlags(uint left, uint right, uint* lower);