guix: use gcc tool wrappers

Summary:
This way, correct --plugin argument are passed through.

This is a prerequisite for LTO (see core#25391).

This is a backport of core#27345

Depends on D15328

Test Plan: contrib/guix.guix-build

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15329