Re-land r335038 "[llvm-exegesis] A mechanism to add target-specific functionality.""

Fix typo: LLVM_NATIVE_ARCH -> LLVM_EXEGESIS_NATIVE_ARCH.

llvm-svn: 335041
diff --git a/llvm/tools/llvm-exegesis/llvm-exegesis.cpp b/llvm/tools/llvm-exegesis/llvm-exegesis.cpp
index 7e2caf3..2ac92965 100644
--- a/llvm/tools/llvm-exegesis/llvm-exegesis.cpp
+++ b/llvm/tools/llvm-exegesis/llvm-exegesis.cpp
@@ -86,6 +86,10 @@
 
 static llvm::ExitOnError ExitOnErr;
 
+#ifdef LLVM_EXEGESIS_INITIALIZE_NATIVE_TARGET
+void LLVM_EXEGESIS_INITIALIZE_NATIVE_TARGET();
+#endif
+
 static unsigned GetOpcodeOrDie(const llvm::MCInstrInfo &MCInstrInfo) {
   if (OpcodeName.empty() && (OpcodeIndex == 0))
     llvm::report_fatal_error(
@@ -120,6 +124,9 @@
 
   llvm::InitializeNativeTarget();
   llvm::InitializeNativeTargetAsmPrinter();
+#ifdef LLVM_EXEGESIS_INITIALIZE_NATIVE_TARGET
+  LLVM_EXEGESIS_INITIALIZE_NATIVE_TARGET();
+#endif
 
   // FIXME: Target-specific filter.
   X86Filter Filter;