[Rootfs] Add GCC 14.2 and make it default for riscv64

GCC 14 has support for the RISV-V Vector Extension 1.0 (RVV1.0), which is
important for packages like OpenBLAS.  `riscv64` is still niche enough that it's
OK to use a more recent, but also more capable, version of GCC as default one.

Author: giordano