Clean up constant usage in linux.rs

[ALSchwalm]

There are a lot of magical constants in https://github.com/mythril-hypervisor/mythril/blob/master/mythril/src/linux.rs

Documentation of many of these field offsets can be found here: https://www.kernel.org/doc/Documentation/x86/boot.txt

We should probably deal with them the same way we do in the ACPI table code, with an offsets mod.