gollvm: add the missing setting of -fcf-protection=none

CL 232742 added a workaround for the upstream bug
https://sourceware.org/bugzilla/show_bug.cgi?id=25921 by adding
-fcf-protection=none c/cxx flag. But this setting seems to have been
omitted. This CL adds it.

Fixes golang/go#38728.

Change-Id: Iabfd4c245fddbf34e2a0c424105c79563d1c0a27
Reviewed-on: https://go-review.googlesource.com/c/gollvm/+/240637
Reviewed-by: Than McIntosh <thanm@google.com>

cmake/modules/AddGollvm.cmake

diff --git a/cmake/modules/AddGollvm.cmake b/cmake/modules/AddGollvm.cmake
index 16c6e4f..9539ee0 100644
--- a/cmake/modules/AddGollvm.cmake
+++ b/cmake/modules/AddGollvm.cmake
@@ -56,6 +56,7 @@
       check_c_source_compiles("#include<stdio.h>\nint main(){printf(\"hello\");\nreturn 0;}" SPLIT_STACK_WORKAROUND)
       if(SPLIT_STACK_WORKAROUND)
         message(STATUS "applying -fcf-protection=none workaround")
+        add_compile_options(-fcf-protection=none)
         set(C_SUPPORTS_SPLIT_STACK 1)
       endif()
     endif()