[dev.link] cmd/link: stop setting ReadOnly attribute in late stage
The ReadOnly attribute was used to do copy on write when applying
relocations to symbols with read-only backing stores. Now that we
always apply relocations in the output buffer (mmap or heap), it
is always writeable. No need to tamper with the ReadOnly
attribute anymore.
Wasm is an exception, where we don't copy symbol contents to the
output buffer first. Do copy-on-write there.
This is in preparation of converting reloc to using the loader.