Compile kernel using GCC 12+ toolchain

[edubart]

Context

The emulator supports the RISC-V 1.12 privileged spec, however this spec was only added starting with GCC 12. Ideally we should use a toolchain that targets the current specification we support to compile to RISC-V, specially the kernel.

Another advantage is to allow enabling "Initialize kernel stack variables at function entry" in kernel config for hardened- kernel. This options is only available only with GCC 12, and makes the Linux kernel more secure.

Possible solutions

We should try to use a RISC-V toolchain already provided by a Linux distribution, such as Ubuntu, instead of maintaining one.