target/ppc: Ensure stcx size matches larx

Differently-sized larx/stcx. pairs can succeed if the starting address
matches. Add a check to require the size of stcx. exactly match the larx
that established the reservation. Use the term "reserve_length" for this
state, which matches the terminology used in the ISA.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20230605025445.161932-2-npiggin@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>