[lldb][AArch64] Use macro for unused var without asserts enabled

82d4f39f342165a92eaa1fe74488158942cf27b4 marked an unused
var of mine (thanks Dmitri Gribenko!). Which reminded me lldb has
a macro just for that purpose.

GitOrigin-RevId: 91098fec960f501c95c6ecdede604484750367e4
1 file changed