jit/x64: make Link() write code to executable memory right away

[quasilyte]

• It's good to avoid excessive []byte -> []byte copying due to the fact that assembler produces a non-executable code first and then it's copied to the destination mapped memory.
• It can be easier to produce proper relocations. (Needs verification.)

We can just add a io.Writer argument to Link() method because of the API that codegen.Compiler provides (it asks to returns a [][]byte for compiler methods).