[PECOFF] Set a proper architecture type to references.

Relocations for x64 object files should have reference type of
KindArch::x86_64.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@200183 91177308-0d34-0410-b5e6-96231b3b80d8
[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.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@200184 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Implement relocations for x86-64.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@200185 91177308-0d34-0410-b5e6-96231b3b80d8