For the record, I attached the failed build log for current core-
updates.

$ ./pre-inst-env guix describe
Git checkout:
  repository: /home/lle-bout/src/guix
  branch: core-updates
  commit: cc6cb6e80a42355147809b4830053a34d1563994

Build command:

$ ./pre-inst-env guix build --target=powerpc64le-linux-gnu bootstrap-
tarballs

The most important bit in the build log is:

configure: error: ***  The compiler must support -mabi=ieeelongdouble
and -mlongdouble simultaneously.

This particular error does not happen with the GCC 7.x and Glibc 2.31
combo.