Support: Use llvm::COFF::BigObjMagic

Use llvm::COFF::BigObjMagic insetad of the string literal.
Also checks the version number.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217633 91177308-0d34-0410-b5e6-96231b3b80d8