target/arm: Permit T32 LDM with single register

target/arm: Permit T32 LDM with single register

For the Thumb T32 encoding of LDM, if only a single register is
specified in the register list this instruction is UNPREDICTABLE,
with the following choices:
 * instruction UNDEFs
 * instruction is a NOP
 * instruction loads a single register
 * instruction loads an unspecified set of registers

Currently we choose to UNDEF (a behaviour chosen in commit
4b222545dbf30 in 2019; previously we treated it as "load the
specified single register").

Unfortunately there is real world code out there (which shipped in at
least Android 11, 12 and 13) which incorrectly uses this
UNPREDICTABLE insn on the assumption that it does a single register
load, which is (presumably) what it happens to do on real hardware,
and is also what it does on the equivalent A32 encoding.

Revert to the pre-4b222545dbf30 behaviour of not UNDEFing
for this T32 encoding.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1799
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230927101853.39288-1-peter.maydell@linaro.org