This also adds riscv to 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.
Note: gcc9 will not work for TARGET_ARCH powerpcspe (not to be confused with IBM Cell, which is a completely different SPE) because they ripped out the target entirely.