Revert D38481 due to missing cmake check for CPU_COUNT

Summary:
This reverts D38481. The change breaks systems with older versions of glibc. It
injects a use of CPU_COUNT() from sched.h without checking to ensure that the
function exists first.

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Subscribers:

llvm-svn: 314922
diff --git a/llvm/tools/llvm-profdata/llvm-profdata.cpp b/llvm/tools/llvm-profdata/llvm-profdata.cpp
index 8e21a7a9..eee2421 100644
--- a/llvm/tools/llvm-profdata/llvm-profdata.cpp
+++ b/llvm/tools/llvm-profdata/llvm-profdata.cpp
@@ -211,8 +211,8 @@
 
   // If NumThreads is not specified, auto-detect a good default.
   if (NumThreads == 0)
-    NumThreads =
-        std::min(hardware_concurrency(), unsigned((Inputs.size() + 1) / 2));
+    NumThreads = std::max(1U, std::min(std::thread::hardware_concurrency(),
+                                       unsigned(Inputs.size() / 2)));
 
   // Initialize the writer contexts.
   SmallVector<std::unique_ptr<WriterContext>, 4> Contexts;