[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
[PECOFF] Use a shorter name for the PECOFF writer member.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@200052 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Set PE32+ magic value if 64 bit.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@200106 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Use a shorter name for the parameter.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@200107 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Emit PE32+ file header.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@200128 91177308-0d34-0410-b5e6-96231b3b80d8