rts: cleanup inlining logic

This patch removes pre-C11 legacy code paths related to
INLINE_HEADER/STATIC_INLINE/EXTERN_INLINE macros, ensure EXTERN_INLINE
is treated as static inline in most cases (fixes #24945), and also
corrects the comments accordingly.

(cherry picked from commit 35a64220)
(cherry picked from commit 241f401d)
(cherry picked from commit 562d9ad7)