arch: add OpenRISC architecture support

arch: add OpenRISC architecture support

Add support for OpenRISC. See here for more details about
OpenRISC http://openrisc.io.

All buildroot included upstream binutils versions are supported.
Gcc support is not upstream, to be able to enable musl C library
support later, we use the branch with musl support.
At the moment it is possible to build a musl based toolchain,
but bootup in Qemu fails.

Gdb is only working to debug bare-metal code, there is no support
for gdbserver/gdb on Linux, yet.

[Peter: drop ?= for GCC_SOURCE]
Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Tested-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>