Upgrade initial GCC build from 4.0.4 to Ekaitz's 4.6.4.1

We currently build GCC 4.0.4 with TCC in Fiwix. This version has limited support for non-x86 architectures (no RISC-V, no ARM64, etc.), and also limits what kernel versions we can build.

As part of his Guix RISC-V bootstrap effort, Ekaitz Zárraga has made a fork of GCC 4.6 (https://github.com/ekaitz-zarraga/gcc), adding support for RISC-V code generation, and making it buildable with TCC. This version has been informally named "4.6.4.1". This newer GCC gives us a chance to upgrade the Linux kernel at least to 4.19, which is supported at least until 2029 by the CIP Project.