Update MemSet/MemCopy helpers on Windows to a faster implementation.

This PR provides an optimized version of JIT_MemSet/JIT_MemCpy assembly helper
functions on 64-bit Windows. JIT_MemSet gets invoked when bytecode Initblk is
executed while JIT_MemCpy gets invoked when bytecode Cpblk is executed.
JIT_MemCpy takes care of both overlap and non-overlap scenarios.
The use of this optimized JIT_MemCpy is extended to Buffer::BlockCopy
and Buffer::InternalBlockCopy by replacing the CRT memmove.

The unit test BlockCopyPerf.cs is used as reference and modified so that the
copy length varies from 0 byte to 520 bytes.
This micro benchmark tests Buffer::BlockCopy (JIT_MemCpy).
The following chart and table show the result on
Intel(R) Core(TM) i7-5960X CPU @ 3.00GHz with 32GB RAM. OS is
Windows 10 Enterprise.

Further details about performance improvements are available at
dotnet#7198

Fixes #7146.

Author: Swaroop Sridhar