Setup GDB in the SDK

Example usage:

```
$ gdb-amd64-usr /build/amd64-usr/usr/lib/systemd/systemd-udevd core.systemd-udevd.0.dc1651f7544c41f4a96e461de0136bfe.9686.1407486660000000 
GNU gdb (Gentoo 7.5.1 p2) 7.5.1
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "--host=x86_64-pc-linux-gnu --target=x86_64-cros-linux-gnu".
For bug reporting instructions, please see:
<http://bugs.gentoo.org/>...
Reading symbols from /build/amd64-usr/usr/lib64/systemd/systemd-udevd...Reading symbols from /build/amd64-usr/usr/lib64/debug/usr/lib64/systemd/systemd-udevd.debug...(no debugging symbols found)...done.
(no debugging symbols found)...done.
[New LWP 9686]

warning: Could not load shared library symbols for linux-vdso.so.1.
Do you need "set solib-search-path" or "set sysroot"?

warning: no loadable sections found in added symbol-file system-supplied DSO at 0x7fff55bfe000
Core was generated by `/usr/lib/systemd/systemd-udevd'.
Program terminated with signal 11, Segmentation fault.
#0  0x00007f036b658cfd in ether_aton_r () from /build/amd64-usr/lib64/libc.so.6
(gdb) bt
#0  0x00007f036b658cfd in ether_aton_r () from /build/amd64-usr/lib64/libc.so.6
#1  0x00007f036c3bb401 in link_config_get ()
#2  0x00007f036c3b8afc in builtin_net_setup_link ()
#3  0x00007f036c3b6692 in udev_builtin_run ()
#4  0x00007f036c3b4b98 in udev_rules_apply_to_event ()
#5  0x00007f036c3ae7b1 in udev_event_execute_rules ()
#6  0x00007f036c3aca8b in worker_new ()
#7  0x00007f036c3abd8a in main ()
(gdb) 
```