Fix static GHCi build (#8270)

When Ryan merged in the atomics branch, he made atomic_inc and
atomic_dec EXTERN_INLINE functions, meaning their definitions
became part of object files in the non-threaded build, instead of being
inlined. As a result, this broke the linker for static GHCi, since it
couldn't properly resolve the now-externally defined functions.

Signed-off-by: Austin Seipp <austin@well-typed.com>