UnmanagedMemoryStream used Interlocked operations to update its state to prevent tearing of 64-bit values on 32-bit platforms. This pattern is expensive in general and it was found to be prohibitively expensive on recent hardware..

This change removes the expensive Interlocked operations and addresses
the tearing issues in alternative way:
- The _length field is converted to nuint that is guaranteed to be
  updated atomically.
- Writes to _length field are volatile to guaranteed the
  unininitialized memory cannot be read.
- The _position field remains long and it has a risk of tearing. It is
  not a problem since tearing of this field cannot lead to buffer
  overruns.

Fixes #93624