[llvm] [AIX][PowerPC] Teach the Threading Library About the Number of Physical Cores on AIX (PR #67683)

[via llvm-commits]

================
@@ -55,6 +55,10 @@
 #include <unistd.h>      // For syscall()
 #endif
 
+#if defined(_AIX)
+#include <sys/systemcfg.h>
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xingxue-ibm wrote:

```suggestion
#include <unistd.h>
```

https://github.com/llvm/llvm-project/pull/67683