Use sched_getaffinity instead of std:🧵:hardware_concurrency.

The issue with std:🧵:hardware_concurrency is that it forwards
to libc and some implementations (like glibc) don't take thread
affinity into consideration.

With this change a llvm program that can execute in only 2 cores will
use 2 threads, even if the machine has 32 cores.

This makes benchmarking a lot easier, but should also help if someone
doesn't want to use all cores for compilation for example.

llvm-svn: 314810

lld/ELF/SyntheticSections.cpp

@@ -2240,8 +2240,8 @@ void MergeNoTailSection::finalizeContents() {
   // operations in the following tight loop.
   size_t Concurrency = 1;
   if (Config->Threads)
-    if (int N = std::thread::hardware_concurrency())
-      Concurrency = std::min<size_t>(PowerOf2Floor(N), NumShards);
+    Concurrency =
+        std::min<size_t>(PowerOf2Floor(hardware_concurrency()), NumShards);
 
   // Add section pieces to the builders.
   parallelForEachN(0, Concurrency, [&](size_t ThreadId) {