target/arm: Define and use new load_cpu_field_low32()

In several places in the 32-bit Arm translate.c, we try to use
load_cpu_field() to load from a CPUARMState field into a TCGv_i32
where the field is actually 64-bit. This works on little-endian
hosts, but gives the wrong half of the register on big-endian.

Add a new load_cpu_field_low32() which loads the low 32 bits
of a 64-bit field into a TCGv_i32. The new macro includes a
compile-time check against accidentally using it on a field
of the wrong size. Use it to fix the two places in the code
where we were using load_cpu_field() on a 64-bit field.

This fixes a bug where on big-endian hosts the guest would
crash after executing an ERET instruction, and a more corner
case one where some UNDEFs for attempted accesses to MSR
banked registers from Secure EL1 might go to the wrong EL.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230424153909.1419369-2-peter.maydell@linaro.org

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