target/openrisc: Replace cpu register array with a function

target/openrisc: Replace cpu register array with a function

The writes to cpu_R[0] are now a race across threads, now that we
do code generation in parallel.  Stage the change by introducing
a function to return the temp for R0.

Reviewed-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>