[llvm-exegesis] Make benchmark pinning actually work

When originally writing this feature up, I apparently completely forgot
to actually make the test exercise it and left an extra exit in the
function implementing the functionality without the appropriate
preprocessor macros around it, causing things to never work. This patch
should fix that.

llvm/test/tools/llvm-exegesis/X86/latency/cpu-pinning.s

@@ -1,5 +1,5 @@
 # REQUIRES: exegesis-can-measure-latency, x86_64-linux
 
-# RUN: llvm-exegesis -mtriple=x86_64-unknown-unknown -mode=latency -opcode-name=ADD64rr -execution-mode=subprocess | FileCheck %s
+# RUN: llvm-exegesis -mtriple=x86_64-unknown-unknown -mode=latency -opcode-name=ADD64rr -execution-mode=subprocess --benchmark-process-cpu=0 | FileCheck %s
 
 # CHECK: - { key: latency, value: {{[0-9.]*}}, per_snippet_value: {{[0-9.]*}}

llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp

@@ -422,8 +422,9 @@ class SubProcessFunctionExecutorImpl
            "Expected getcpu call to succeed.");
     assert(static_cast<int>(CurrentCPU) == CPUToUse &&
            "Expected current CPU to equal the CPU requested by the user");
+#else
+    exit(ChildProcessExChildProcessExitCodeE::SetCPUAffinityFailed));
 #endif // defined(__x86_64__) && defined(SYS_getcpu)
-    exit(ChildProcessExitCodeE::SetCPUAffinityFailed);
   }
 
   Error createSubProcessAndRunBenchmark(