Make SHIM_LOCK_GUID a first-class object with a symbol.

Right now the CA is checking if shim builds expose a particular version
of the shim protocol.  To do this, they're looking for SHIM_LOCK_GUID's
value in the resulting binary.

Currently, with SHIM_LOCK_GUID as a macro that gets assigned to local
variables, that means they have to compensate for mov instructions mixed
in with the actual value.  This is completely absurd, so promote it to a
first-class object with a symbol to make it both easy to find and
continuous.

Signed-off-by: Peter Jones <pjones@redhat.com>
Made TODO represent the present.

Signed-off-by: Peter Jones <pjones@redhat.com>