asmlinkage: Make main_extable_sort_needed visible

main_extable_sort_needed is used by the build system and needs
to be a normal ELF symbol. Make it visible so that LTO
does not remove or mangle it.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Link: http://lkml.kernel.org/r/1391845930-28580-8-git-send-email-ak@linux.intel.com


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>