arm64: Make stxr_status an input operand in vm_do_cheri_revoke()

If the capability under test changes between the initial load (done to
clear the tag before storing it) and the load-link, the initial
comparison fails and the inline asm block is supposed to leave
stxr_status set to 2.  However, the lack of an input constraint meant
that the compiler simply elided the initialization of stxr_status, which
could lead to an infinite loop.

Thus, make stxr_status an input operand as well as an output operand.

sys/arm64/arm64/cheri_revoke_machdep.c

@@ -159,7 +159,7 @@ vm_do_cheri_revoke(int *res, const struct vm_cheri_revoke_cookie *crc,
 #endif
 		  : [stxr_status] "=r" (stxr_status),
 		    [cscratch] "=&C" (cscratch), [cutr] "+C" (cutr)
-		  : [cut] "C" (cut), [cutp] "C" (cutp)
+		  : [cut] "C" (cut), [cutp] "C" (cutp), "r" (stxr_status),
 		  : "memory");
 
 		/* stxr returns 0 on success */