http://mail-index.netbsd.org/source-changes/2020/04/16/msg116278.html
The reasoning turned out to be wrong; __KERNEL_RCSID() in header files
does *not* overwrite RCSID in main source files. The real problem is that
it inserts its RCSID into *every* object files. However, it can be still
useful even if heavily duplicated.
overwriting RCSID in main source files.
XXX
The first argument of __KERNEL_RCSID() is neglected for ELF. If we wish
to have RCSID of header files in kernel binary, we need something like
__FBSDID() macro in FreeBSD.