Now that cibuildwheel and PyPI support riscv64, we can start building
riscv64 wheels.

Because there is no native riscv64 runner available, this PR adds a
QEMU-based riscv64 job to the cibuildwheel workflow.

Signed-off-by: Julien Stephan <jstephan@baylibre.com>

Similar to msgpack#665, just a return value check to propagate the error in
case one happens (instead of silently ignoring it).

Note that as opposed to the previous lines, we don't need to
`PyErr_SetString` since `unpack_callback_uint32` calls
`PyLong_FromSize_t` which itself should set whatever Python error is
relevant; we just need to make it clear to the caller that an error
occurred.