MC: Emit COFF section flags in the "proper" order

MC: Emit COFF section flags in the "proper" order

COFF section flags are not idempotent:
  'rd' will make a read-write section because 'd' implies write
  'dr' will make a read-only section because 'r' disables write

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228490 91177308-0d34-0410-b5e6-96231b3b80d8