Fix endianness in ObjectFile::CopyData

ObjectFile::CopyData is used to copy a block of target memory to the
caller's buffer (e.g. for "memory read").  This should be a straight
memcpy, and not byte-swapped if the target and host have different
endianness.

Add a new DataExtractor::CopyData() method that performs this straight
copy and use it in ObjectFile::CopyData().