Move constant pools in compiled code out of the .text section

Today Cranelift's constant-pools are located in the .text section of the executable, typically located after the function itself. While convenient for code generation this exposes a possible attack vector in Wasmtime where it's trivial to put a "gadget" somewhere in memory. For example using a sequence of `v128.const` it would be pretty easy to assemble "machine code" at the end of a function. In the face of a bug in Cranelift this could make it possibly easier to amplify into a sandbox escape perhaps.

As a defense-in-depth measure we should try to move the constant pools out of the .text section and into a .data or otherwise read-only section. (not writable or executable). This won't be trivial to do due to the fact that relocations from the text section point at the data section and the relocation range may not always be large enough for the entire text section. Regardless though I wanted to file an issue about this idea.