[gn] port 8e8692a (RISCV support for llvm-exegesis)

llvm/utils/gn/secondary/llvm/lib/Target/targets_with_exegesis.gni

@@ -3,7 +3,7 @@ import("//llvm/lib/Target/targets.gni")
 targets_with_exegesis = []
 foreach(target, llvm_targets_to_build) {
   if (target == "AArch64" || target == "Mips" || target == "PowerPC" ||
-      target == "X86") {
+      target == "RISCV" || target == "X86") {
     targets_with_exegesis += [ target ]
   }
 }

llvm/utils/gn/secondary/llvm/tools/llvm-exegesis/lib/RISCV/BUILD.gn

@@ -0,0 +1,14 @@
+static_library("RISCV") {
+  output_name = "LLVMExegesisRISCV"
+  deps = [
+    "//llvm/lib/CodeGen",
+    "//llvm/lib/IR",
+    "//llvm/lib/Support",
+
+    # Exegesis reaches inside the Target/RISCV tablegen internals and must
+    # depend on these Target/RISCV-internal build targets.
+    "//llvm/lib/Target/RISCV/MCTargetDesc",
+  ]
+  sources = [ "Target.cpp" ]
+  include_dirs = [ "//llvm/lib/Target/RISCV" ]
+}