urcu/uatomic/riscv: Mark RISC-V as broken

Implementations of some atomic operations of GCC for RISC-V are
insufficient for sequential consistency. For this reason Userspace RCU
is currently marked as `broken' for RISC-V with GCC. However, it is
still possible to use other toolchains.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104831 for details.

For now, we mark every version of GCC as unsupported. Distribution
package maintainers will have to cherry-pick the relevant patches in GCC
then remove the #error in Userspace RCU if they want to support it.

As for us, we will incrementally add specific versions of GCC that have
fixed the issue whenever new stable releases are made from the GCC
project.

Change-Id: I2cd7c8f12068628b845a096e03f5f8100eacbe43
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>