core: Fix backtrace for gcc 6
core: Fix backtrace for gcc 6

GCC 6 warns when we look at any stack frame other than our own, ie any
argument to __builtin_frame_address other than zero.

Signed-off-by: Joel Stanley <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@jms.id.au>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>
(cherry picked from commit 793f6f5b32c96f2774bd955b6062c74a672317ca)

qemu/skiboot,open-power/skiboot,qemu/skiboot,mikey/skiboot,qemu/skiboot,open-power/skiboot,shenki/skiboot,mikey/skiboot,shenki/skiboot,legoater/skiboot,open-power/skiboot,qemu/skiboot,open-power/skiboot,mikey/skiboot,qemu/skiboot,shenki/skiboot,legoater/skiboot,legoater/skiboot,shenki/skiboot,legoater/skiboot,stewart-ibm/skiboot,legoater/skiboot,stewart-ibm/skiboot,stewart-ibm/skiboot,shenki/skiboot,open-power/skiboot