Fix symbol versioning on architectures with symbol prefixes. Although
the build was free from warnings the actual modules still wouldn't load
as the ____versions table contained unprefixed symbol names, which were
being compared against the prefixed symbol names when checking the
symbol versions.

This is fixed by modifying modpost to add the symbol prefix to the
____versions table it outputs (Modules.symvers still contains unprefixed
symbol names). The check_modstruct_version() function is also fixed as
it checks the version of the unprefixed "module_layout" symbol which
would no longer work.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jonathan Kliegman <kliegs@chromium.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (use VMLINUX_SYMBOL_STR)