User mode needs to be able to read this value in
compiler generated function prologues/epilogues.
Special handling in init.c for arches that use
_data_copy. This happens before _Cstart() gets
called. We need to make sure that the compiler
stack canary checks in _data_copy itself do not
fail.
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>