cranelift-module: Support marking data object as read-only C string

#### Feature

See title

#### Benefit

This would allow the linker to deduplicate the strings. In addition some linker don't like mixing attributes between sections that get merged, so if the user of cg_clif specified that the data object needs to end up in `__TEXT/__cstring`, we will need to respect the `cstring_literals` section type likely specified by the user too.

#### Implementation

cranelift-jit can ignore this and cranelift-object can use `SectionKind::ReadOnlyString`.