Buildworld fatal error: 'byteswap.h' file not found on armv7

As an experiment I've tried to build -current and 14.2-stable
on a Pi2 v1.1 running 12.4-stable. I know it's not considered
good practice to hop that far, but I wanted to see for myself.

I thought it would at least compile, since I have another Pi2v1.1
which has been tracking -current and is presently up-to-date.

However, the build fails in both buildworld and toolchain, with
/usr/src/contrib/file/src/apprentice.c:54:10: fatal error: 'byteswap.h' file not found
#include <byteswap.h>
         ^~~~~~~~~~~~

The missing file is actually present:
root@pelorus:/usr/src # find . -name byteswap.h -depth -print
./contrib/llvm-project/libcxx/include/__bit/byteswap.h
./contrib/ofed/include/byteswap.h
./include/byteswap.h
root@pelorus:/usr/src # 

It's a little hard to see how using 12.4 might cause this sort of error.
The source directory is a fresh clone. 

Is this an expected failure mode? If there's an easy workaround I'd like
to try, but if not it's no big deal, I'll just reflash the microSD.

Thanks for reading, and any insights. 

bob prohaska