Investigate using JIT_Memset over Memset in Span.Clear

[ahsonkhan]

From PR dotnet/coreclr#9598

dotnet/coreclr#9598 (comment)

Can JIT_MemSet be used and does it offer any improvement? (Not sure what path it gets used on) dotnet/coreclr#7198

Worth trying. I would call JIT_MemSet from the implementation of the PInvoke if it is available and faster. I would not call it directly from here. The condition for when it is available and faster potentially involves architectures, OS versions, and compiler/C-runtime versions. It is better for such condition to be on C++ side.