fix "Missing break in switch" coverity reports

Many of these are marked as "intentional/fix required" because they
just need adding a fall through comment.  This is exactly what this
patch does, except for target/mips/translate.c where it is easier to
duplicate the code, and hw/audio/sb16.c where I consulted the DOSBox
sources and decide to just remove the LOG_UNIMP before the fallthrough.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/arm/helper.c

@@ -12331,6 +12331,7 @@ int arm_rmode_to_sf(int rmode)
         /* FIXME: add support for TIEAWAY and ODD */
         qemu_log_mask(LOG_UNIMP, "arm: unimplemented rounding mode: %d\n",
                       rmode);
+        /* fall through for now */
     case FPROUNDING_TIEEVEN:
     default:
         rmode = float_round_nearest_even;