gdb: add Lancelot Six as maintainer of the AMD GPU port

Lancelot has accepted to take the role of maintainer for the AMD GPU
port.  The AMD GPU port (amdgpu as I've written in the MAINTAINERS file)
is an umbrella term for everything needed to make this work: the amdgcn
arch, the amd-dbgapi target, solib-rocm, etc.

Thanks for accepting the role, and congratulations!

Change-Id: I4c898042fda49b45dcb0d54ca94731bb57287f71