Make Encoding.Unicode.GetString faster on 64-bit platforms (#18263)

Make Encoding.Unicode.GetString faster on 64-bit platforms (#18263)

UnicodeEncoding was taking fast path only when both source and destination were 8-byte aligned. It was not
the case for Unicode.GetString.

The fix is to just check the source alignment as fast path precondition, and use unaligned write for destination.

Also removed alternative fast path that is not needed anymore, tweaked local variable assignments for better
register allocation, and fixed performance issue introduced by recent big endian portability fix.