target/arm: Add v8M stack checks for Thumb2 LDM/STM

target/arm: Add v8M stack checks for Thumb2 LDM/STM

Add the v8M stack checks for:
 * LDM (T2 encoding)
 * STM (T2 encoding)

This includes the 32-bit encodings of the instructions listed
in v8M ARM ARM rule R_YVWT as
 * LDM, LDMIA, LDMFD
 * LDMDB, LDMEA
 * POP (multiple registers)
 * PUSH (muliple registers)
 * STM, STMIA, STMEA
 * STMDB, STMFD

We perform the stack limit before doing any other part
of the load or store.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-id: 20181002163556[email protected]