MC: Treat ReadOnlyWithRel and ReadOnlyWithRelLocal as ReadOnly for COFF

A problem with our old behavior becomes observable under x86-64 COFF
when we need a read-only GV which has an initializer which is referenced
using a relocation: we would mark the section as writable.  Marking the
section as writable interferes with section merging.

This fixes PR21009.

llvm-svn: 218179