target/arm: Don't trap WFI/WFE for M profile

M profile cores can never trap on WFI or WFE instructions. Check for
M profile in check_wfx_trap() to ensure this.

The existing code will do the right thing for v7M cores because
the hcr_el2 and scr_el3 registers will be all-zeroes and so we
won't attempt to trap, but when we start setting ARM_FEATURE_V8
for v8M cores the v8A handling of SCTLR.nTWE and .nTWI will not
give the right results.

Backports commit 0e2845689ebdb4ea7174f96f6797e2d8942bd114 from qemu

qemu/target/arm/op_helper.c

@@ -369,6 +369,11 @@ static inline int check_wfx_trap(CPUARMState *env, bool is_wfe)
     int cur_el = arm_current_el(env);
     uint64_t mask;
 
+    if (arm_feature(env, ARM_FEATURE_M)) {
+        /* M profile cores can never trap WFI/WFE. */
+        return 0;
+    }
+
     /* If we are currently in EL0 then we need to check if SCTLR is set up for
      * WFx instructions being trapped to EL1. These trap bits don't exist in v7.
      */