Make the NCG distinguish between the read-only data section and the
"relocatable read-only data" section.
Read-only data is supposed to be _really_ read-only, whereas "relrodata"
can have relocations, but should not be modified by the program at runtime.

For Linux, put relrodata into ".data" by default, as the dynamic linker
tends to do evil things to avoid relocating things in read-only sections.