[Concurrency] Disable flaky test (swiftlang#40875)

Author: philipturner

test/Sanitizers/tsan/racy_async_let_fibonacci.swift

@@ -15,7 +15,8 @@
 // UNSUPPORTED: OS=watchos
 
 // rdar://86825277
-// UNSUPPORTED: CPU=arm64e
+// SR-15805
+// UNSUPPORTED: CPU=arm64 || CPU=arm64e
 
 func fib(_ n: Int) -> Int {
   var first = 0