[CMake] Split -gx strip flag into -g -x

[phosek]

Summary

llvm-strip doesn't handle -gx spelling, so we need to split these
as two separate flags.

llvm/cmake/modules/AddLLVM.cmake

function(llvm_externalize_debuginfo name)
 
   if(NOT LLVM_EXTERNALIZE_DEBUGINFO_SKIP_STRIP)
     if(APPLE)
       if(NOT CMAKE_STRIP)
         set(CMAKE_STRIP xcrun strip)
       endif()
       set(strip_command COMMAND ${CMAKE_STRIP} -Sxl $<TARGET_FILE:${name}>)
     else()
-      set(strip_command COMMAND ${CMAKE_STRIP} -gx $<TARGET_FILE:${name}>)
+      set(strip_command COMMAND ${CMAKE_STRIP} -g -x $<TARGET_FILE:${name}>)
     endif()
   endif()
 
   if(APPLE)
     if(CMAKE_CXX_FLAGS MATCHES "-flto"
       OR CMAKE_CXX_FLAGS_${uppercase_CMAKE_BUILD_TYPE} MATCHES "-flto")
 
       set(lto_object ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}/${name}-lto.o)