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"