target/arm: Fix SCTLR_B test for TCGv_i64 load/store

target/arm: Fix SCTLR_B test for TCGv_i64 load/store

Just because operating on a TCGv_i64 temporary does not
mean that we're performing a 64-bit operation.  Restrict
the frobbing to actual 64-bit operations.

This bug is not currently visible because all current
users of these two functions always pass MO_64.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210419202257.161730-14-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>