[Concurrency] Limit futures test to x86_64 for now.

We have not yet implemented ptrauth support for the async calling
convention, so this test is crashing on arm64e. Limit it to x86_64
until that comes online.

Fixes rdar://71592154.

Author: DougGregor

test/Concurrency/Runtime/basic_future.swift

@@ -3,6 +3,7 @@
 // REQUIRES: executable_test
 // REQUIRES: concurrency
 // REQUIRES: OS=macosx
+// REQUIRES: CPU=x86_64
 
 import Dispatch