MC: ReadOnlyWithRel section kinds should map to rdata in COFF

Don't consider ReadOnlyWithRel as a writable section in COFF, they
really belong in .rdata.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218268 91177308-0d34-0410-b5e6-96231b3b80d8