Default activeProcessorCount to 1, not 0.

After a rather tedious debugging session trying to figure out why
swiftpm-bootstrap appeared to be deadlocked, this turned out to be the
culprit. Perhaps this should be #error instead, but for now, set a
sensible default.

Author: 3405691582

Sources/FoundationEssentials/ProcessInfo/ProcessInfo.swift

@@ -481,7 +481,7 @@ extension _ProcessInfo {
         GetSystemInfo(&sysInfo)
         return sysInfo.dwActiveProcessorMask.nonzeroBitCount
 #else
-        return 0
+        return 1
 #endif
     }