target/arm: Make sure M-profile FPSCR RES0 bits are not settable

Enforce that for M-profile various FPSCR bits which are RES0 there
but have defined meanings on A-profile are never settable. This
ensures that M-profile code can't enable the A-profile behaviour
(notably vector length/stride handling) by accident.

Backports commit 5bcf8ed9401e62c73158ba110864ee1375558bf7 from qemu

qemu/target/arm/vfp_helper.c

@@ -105,6 +105,14 @@ void HELPER(vfp_set_fpscr)(CPUARMState *env, uint32_t val)
         val &= ~FPCR_FZ16;
     }
 
+    if (arm_feature(env, ARM_FEATURE_M)) {
+        /*
+         * M profile FPSCR is RES0 for the QC, STRIDE, FZ16, LEN bits
+         * and also for the trapped-exception-handling bits IxE.
+         */
+        val &= 0xf7c0009f;
+    }
+
     /*
      * We don't implement trapped exception handling, so the
      * trap enable bits, IDE|IXE|UFE|OFE|DZE|IOE are all RAZ/WI (not RES0!)