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

Fix typo: LLVM_NATIVE_ARCH -> LLVM_EXEGESIS_NATIVE_ARCH.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335041 91177308-0d34-0410-b5e6-96231b3b80d8
11 files changed