[PECOFF] Accept /machine:x64 option.

This is the first patch to support PE32+ format, which is the image format
to use 64 bit address space on Windows/x86-64.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@200029 91177308-0d34-0410-b5e6-96231b3b80d8