bitarithm: Move extern const out of static inline functions

[chrysn]

Contribution description

Work around a transpilation issue in bitarithm.h that'd be triggered if bitarithm.h were included through any header file transpiled through C2Rust.

This is a temporary workaround that unblocks cases when this would make Rust builds fail; once a newer C2Rust version is in CI, it can be reverted.

Testing procedure

• Make bitarithm.h visible to C2Rust, eg. by working from nanocoap_vfs: add nanocoap_vfs_put() #17962, or more easily by applying

diff --git a/core/lib/include/irq.h b/core/lib/include/irq.h
index 25b609f95b..b84bf6819f 100644
--- a/core/lib/include/irq.h
+++ b/core/lib/include/irq.h
@@ -24,6 +24,7 @@
 
 #include <stdbool.h>
 #include "cpu_conf.h"
+#include <bitarithm.h>
 
 #ifdef __cplusplus
 extern "C" {

• Build the rust-hello-world example for BOARD=samr30-xpro -- before, that failed inside the Rust build parts (riot-sys), now it builds.

Issues/PRs references

• Unblocks: nanocoap_vfs: add nanocoap_vfs_put() #17962
• Workaround-For: Extern items referenced in two functions cause "name not declared" error immunant/c2rust#423

[kaspar030]

ACK.

edit code change itself looks fine (just shuffling around). I did not test whether that solves the linked issue, trusting @chrysn's testing here.

[chrysn]

Force-pushed a fix that adds (@private) documentation to satisfy the formal documentation requirement without actually including it in user visible docs.