[PECOFF] Set a proper architecture type to references.

Relocations for x64 object files should have reference type of
KindArch::x86_64.
[PECOFF] Fix PE/COFF optional header size for PE32+.

The optional header for PE32+ is a bit larger than PE32 because some of
its fields are extended to 8 bytes.
[PECOFF] Implement relocations for x86-64.