[System.Core] Using more reference source files.

[System.Core] Using more reference source files.

Changed MemoryMappedFile related code to use reference source
MemoryMappedViewAccessor and MemoryMappedViewStream.

This change was motivated by segmentations faults while dnmv code was
calling a CoreFX class that uses reflection to call MemoryMappedFile
code.

Now that we are using more code from reference sources there should be
no problem.

Fixes #30741 and #30825.