ensure W^X mapping of JIT code

[JonasKruckenberg]

As described in this blog post: https://jandemooij.nl/blog/wx-jit-code-enabled-in-firefox/
pages containing JIT code need to be writable during creation, but should never be writable during execution. If we never want to touch the code pages again, it's trivial to do a simple page permission swap and call it good.
If we need to patch code at runtime (e.g. in case the VM becomes two-tiered) we need to implement a solid system for doing so.

1. Spidermonkey apparently uses a guard object that allows for temporary mutation of code pages and will reset to RX on drop.
2. A different approach we might consider for k23 is dual-mapping code regions, once as RX in user space and once as RW in kernel space.