libsel4-sys: bindgen not expanding functional macros

[jlamb-at-polysync]

When building for arm-sel4-helios, we get an error in the root-task, related to undefined/missing type seL4_WordBits.

cargo fel4 build --target arm-sel4-helios:

error[E0425]: cannot find value `seL4_WordBits` in this scope
  --> src/bin/root-task.rs:57:13
   |
57 |             seL4_WordBits.into(),
   |             ^^^^^^^^^^^^^ did you mean `seL4_GuardBits`?

This is because libsel4 is using a functional macro in (ARM configs, ie libsel4/sel4_arch_include/aarch32/sel4/sel4_arch/constants.h):

#define seL4_WordBits (sizeof(seL4_Word) * 8)

We don't see this with the x86-64 target because it uses #define seL4_WordBits 64.

Seems to be related to this bindgen issue: rust-lang/rust-bindgen#753