target/arm: Implement new v8.1M VLLDM and VLSTM encodings

target/arm: Implement new v8.1M VLLDM and VLSTM encodings

v8.1M adds new encodings of VLLDM and VLSTM (where bit 7 is set).
The only difference is that:
 * the old T1 encodings UNDEF if the implementation implements 32
   Dregs (this is currently architecturally impossible for M-profile)
 * the new T2 encodings have the implementation-defined option to
   read from memory (discarding the data) or write UNKNOWN values to
   memory for the stack slots that would be D16-D31

We choose not to make those accesses, so for us the two
instructions behave identically assuming they don't UNDEF.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201119215617.29887-21-peter.maydell@linaro.org