[gn build] Manually port 8f03e1a

Author: aeubanks

llvm/utils/gn/secondary/clang/test/BUILD.gn

@@ -68,6 +68,7 @@ write_lit_config("lit_site_cfg") {
     "CMAKE_C_COMPILER=cc",
     "ENABLE_BACKTRACES=1",
     "ENABLE_SHARED=0",
+    "LLVM_EXPERIMENTAL_KEY_INSTRUCTIONS=0",
     "LLVM_EXTERNAL_LIT=",
     "LLVM_HOST_TRIPLE=$llvm_current_triple",
     "LLVM_INCLUDE_SPIRV_TOOLS_TESTS=0",