This also adds riscv to the GCC tinderbox.
- rG9dfbda254c20: Switch to GCC 9 for the GCC tinderbox.
- make USE_GCC_TOOLCHAINS=yes tinderbox
riscv doesn't link due to jemalloc using atomics on a bool that needs __atomic_exchange_1. That isn't new with GCC 9 though and would fail with riscv-xtoolchain-gcc as well.
powerpc64 dies with an ICE.
32-bit arm is now the only architecture without a GCC toolchain.