powerpc: add support for syscall stack randomization

Add support for adding a random offset to the stack while handling
syscalls. This patch uses mftb() instead of get_random_int() for better
performance.

In order to avoid unconditional stack canaries on syscall entry (due to
the use of alloca()), also disable stack protector to avoid triggering
needless checks and slowing down the entry path. As there is no general
way to control stack protector coverage with a function attribute, this
must be disabled at the compilation unit level.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220701082435.126596-3-xiujianfeng@huawei.com