[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