elflink: Move more code from core/ into ldlinux

elflink: Move more code from core/ into ldlinux

ldlinux now contains all the code necessary to load and execute
modules, none is contained in the core.

This change also allows us to change the spawn_load() prototype and to
push the job of processing arguments to executable functions (e.g. the
contents of char *argv[] as passed to a module's main function) into
ldlinux/execute.c instead of doing it in spawn_load(). Moving it into
ldlinux/ makes sense because the only core user of spawn_load() is
load_env32() and we don't require any sort of argument processing in
that path.

Signed-off-by: Matt Fleming <matt.fleming@linux.intel.com>