arm64: Initialize stxr_status in asm 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.

This could be fixed by making stxr_status an input operand, but it seems
safer to just initialize it in the asm block.

sys/arm64/arm64/cheri_revoke_machdep.c

@@ -140,9 +140,8 @@ vm_do_cheri_revoke(int *res, const struct vm_cheri_revoke_cookie *crc,
 		 * stxr returns 0 or 1, so use a value of 2
 		 * to indicate that it was not executed.
 		 */
-		stxr_status = 2;
-
 		__asm__ __volatile__ (
+		        "mov %[stxr_status], #2\n\t"
 #ifndef __CHERI_PURE_CAPABILITY__
 			"bx #4\n\t"
 			".arch_extension c64\n\t"