Fix GCC 7 -Wimplicit-fallthrough warnings.

dynasm/dasm_arm.h

@@ -254,6 +254,7 @@ void dasm_put(Dst_DECL, int start, ...)
       case DASM_IMMV8:
 	CK((n & 3) == 0, RANGE_I);
 	n >>= 2;
+	/* fallthrough */
       case DASM_IMML8:
       case DASM_IMML12:
 	CK(n >= 0 ? ((n>>((ins>>5)&31)) == 0) :
@@ -371,6 +372,7 @@ int dasm_encode(Dst_DECL, void *buffer)
 	  break;
 	case DASM_REL_LG:
 	  CK(n >= 0, UNDEF_LG);
+	  /* fallthrough */
 	case DASM_REL_PC:
 	  CK(n >= 0, UNDEF_PC);
 	  n = *DASM_POS2PTR(D, n) - (int)((char *)cp - base) - 4;