The CI server successfully built linux-libre 5.7 for x86_64, i686, and
aarch64:

http://ci.guix.gnu.org/search?query=spec%3Akernel-updates+linux-libre-5.7

I guess that it doesn't build kernels for armhf, since it appears that
nothing was attempted.

Anyways, I pushed the update as 843344273c6a587b8e6c84d8406500fd64d8908a

Please report any problems!