Unconditionally define __stack_chk_guard [BZ #26817]

Unconditionally define __stack_chk_guard [BZ #26817]

__stack_chk_guard is currently unavailable on architectures which define
THREAD_SET_STACK_GUARD, so {gcc,clang} -fstack-protector
-mstack-protector-guard=global will fail to link due to the undefined
symbol.

Define __stack_chk_guard to make -mstack-protector-guard=global compiled
user programs work. `#define THREAD_SET_STACK_GUARD` code is moved
outside of `#ifndef __ASSEMBLER__` so that cpp can preprocess
elf/Versions.