xtensa: drop EXPORT_SYMBOL for common_exception_return

common_exception_return used to be referenced from the oprofile code,
but since oprofile support removal in the commit
a848bf1d ("arch: xtensa: Remove CONFIG_OPROFILE support")
it is no longer supposed to be used by modules.
Drop EXPORT_SYMBOL for common_exception_return.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>