Good idea to create a fork of the repo.

I've not seen that specific failure before, unfortunately.

I've checked my sysroot directory, running (essentially)
$ icdiff <(find aarch64-sysroot/ | sort) <(sort sysroot.contents)
and the only difference is extra files your side, none of which should
AFAIU make a difference, but here they are (cleaned up):

usr/include/dev/isa/cs4231var.h
usr/include/dev/mii/xmphyreg.h
usr/include/dev/pci/if_icereg.h
usr/include/dev/pci/if_icevar.h
usr/include/dev/pci/pciide_opti_reg.h
usr/include/openssl/conf_api.h
usr/include/openssl/pem2.h
usr/include/openssl/ssl2.h
usr/include/openssl/ssl23.h
usr/include/openssl/ui_compat.h
usr/include/openssl/whrlpool.h
usr/include/ufs/ffs/softdep.h
usr/lib/libc.so.100.0
usr/lib/libc.so.100.1
usr/lib/libc.so.100.2
usr/lib/libc.so.99.0
usr/lib/libcrypto.so.53.0
usr/lib/libcrypto.so.54.0
usr/lib/libperl.so.23.0
usr/lib/libradius.so.1.0
usr/lib/libsndio.so.7.2
usr/lib/libssl.so.56.0
usr/lib/libssl.so.57.0
usr/lib/libtls.so.29.0
usr/lib/libtls.so.30.0
usr/lib/libz.so.7.0
usr/local/include/uniconv.h

I doubt any of those are the problem, but if I had to pick one as
suspicious, the various extra libc versions catch my eye (though you
also had the one version I had). But I don't see that resulting in a
failure to find float.h. It's definitely in the sysroot, and I'm sure
it's in your amd64 system's /usr/include.

The uniconv.h is also curious, though not really suspect. I don't have
it in my sysroot.

Anyway, I've cloned your repo, given it a sysroot.tar.gz I have, and
I'll let it run. I'll report back in a couple hours hopefully.

Cheers,
Habib