I haven't been able to test it though; when attempting to rebuild gcc, I
get a stack overflow before the build process even starts (seems like a
guix problem -- I'll try to git pull & rebuild):

--8<---------------cut here---------------start------------->8---
./pre-inst-env guix build -K gcc
allocate_stack failed: Cannot allocate memory
Warning: Unwind-only `stack-overflow' exception; skipping pre-unwind handler.
allocate_stack failed: Cannot allocate memory
Warning: Unwind-only `stack-overflow' exception; skipping pre-unwind handler.
allocate_stack failed: Cannot allocate memory
Warning: Unwind-only `stack-overflow' exception; skipping pre-unwind handler.
allocate_stack failed: Cannot allocate memory
Warning: Unwind-only `stack-overflow' exception; skipping pre-unwind handler.
--8<---------------cut here---------------end--------------->8---

Maxim