Make the platform dynamic

[naure]

Currently, platform details (e.g. special memory addresses) are constants, and guest programs must target it.

It’s possible to upgrade these constants to parameters chosen by guest programs. These parameters can be given in ELF files and/or CLI arguments.

To do it, replace all usages of the CENO_PLATFORM constant into a platform: Platform argument, like for example VMState.platform does. It might be harder than it sounds; did not try yet.

Refs: platform.rs, linker script, etc.

Related: #215, #378, #476.