target/arm: Implement CLRM instruction

target/arm: Implement CLRM instruction

In v8.1M the new CLRM instruction allows zeroing an arbitrary set of
the general-purpose registers and APSR.  Implement this.

The encoding is a subset of the LDMIA T2 encoding, using what would
be Rn=0b1111 (which UNDEFs for LDMIA).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201119215617.29887-6-peter.maydell@linaro.org