!20 (merged) and !21 (merged) added some code to support Zebu running on sel4-rumprun. However, it does not run on sel4-rumprun yet, due to rodal does not support sel4-rumprun.

The changes mainly address these issues:

• removed usage of dynamic libraries (dlopen, dlsym, etc) as sel4-rumprun does not support dynamic linking.
• rewrote some testcases to avoid using dynamic libraries.
• added feature guard sel4-rumprun for OS dependent code. Feature guard is used instead of OS guard as Rust does not correctly recognise sel4-rumprun.
• added feature guard sel4-rumprun-target-side for two-stage cross compilation.

Problems with the changes:

• it doesn't actually run on rumprun (rodal uses dlsym).
• the changes currently have massive code duplication instead of reusing OS/Target dependent code for Linux/x86_64. And duplicated code is not maintained when the original code changes.
• there is some hard-coded path for running Zebu on sel4-rumprun.
• using feature guard [#cfg(feature = "sel4-rumprun")] instead of a proper OS guard [#cfg(target_os = "sel4-rumprun")] makes OS dependent code quite unreadable. For example, for linux code, we have to do

[#cfg(not(feature = "sel4-rumprun))]
[#cfg(target_os = "linux")]
... // linux dependent code

• there is no document on how to setup environment and run Zebu on sel4-rumprun.

These should be addressed if we want to properly support Zebu on sel4-rumprun.