add user-defined load/unload hook support

This commit enables the ability to create user-defined hooks as part of
the normal code patch that can do preparatory work for the application
of the patch.  This work could include, but is not limited to, changing
data structure semantics.

The user may define a new function as part of the patch and mark it as a
load-time or unload-time hook with the kpatch_load_hook() and
kpatch_unload_hook() macros.  These macros are in an include file that
gets copied into the source tree at include/linux/kpatch-hooks.h at
patch build time. The signature for both hooks is "int kpatch_hook(void)".

For now, the return code is ignored.  The hooks may not fail.  They also
run in stop_machine() context and may not sleep.  These hooks, more or
less, must follow all the rules of interrupt context code.
Since we have no plans to check hook errors, should we just make them return void?