PE header for x64

For 64-bit executables/PE files, there are a couple of changes in the PE header offsets.

1. Don’t consider the size of the OptionalHeader as 0x74, instead use the “SizeOfOptionalHeader” from the _IMAGE_FILE_HEADER.
2. There is no longer a BaseOfData field, instead ImageBase is 8 bytes long. More details on _IMAGE_OPTIONAL_HEADER64 you can found here.