Building for ELFv2

[Cameron Berkenpas]

Hello,

How can one build FreeBSD 13.0-CURRENT for ELFv2 support?

I've installed xtoolchain-llvm90, and I run:
CROSS_TOOLCHAIN=llvm90 make buildenv

 From here buildworld fails pretty early on with:
/usr/src/sys/contrib/libnv/nvpair.c:33:10: fatal error: 'sys/cdefs.h' 
file not found

I can add "-I/usr/include" and so on to /etc/src.conf to hack around 
this, it just goes a bit further and finds other headers missing.

As far as I can tell, $SYSROOT (/usr/obj/usr/src/powerpc.powerpc64/tmp) 
isn't being properly setup properly which is resulting in missing headers.

I've done some searching and haven't really found anything too helpful. 
But perhaps I just don't know what to look for.

Thanks!