[bazel] Turn on EXPERIMENTAL_KEY_INSTRUCTIONS

It has been introduced in #131344 and turned on at #144324

utils/bazel/llvm-project-overlay/llvm/config.bzl

@@ -112,6 +112,8 @@ llvm_config_defines = os_defines + builtin_thread_pointer + select({
     "LLVM_VERSION_MINOR={}".format(LLVM_VERSION_MINOR),
     "LLVM_VERSION_PATCH={}".format(LLVM_VERSION_PATCH),
     r'LLVM_VERSION_STRING=\"{}\"'.format(PACKAGE_VERSION),
+    # Set globally in HandleLLVMOptions.cmake
+    "EXPERIMENTAL_KEY_INSTRUCTIONS",
     # These shouldn't be needed by the C++11 standard, but are for some
     # platforms (e.g. glibc < 2.18. See
     # https://sourceware.org/bugzilla/show_bug.cgi?id=15366). These are also