x86/power: Support objtool validation in hibernate_asm_64.S

The OBJECT_FILES_NON_STANDARD annotation is used to tell objtool to
ignore a file.  File-level ignores won't work when validating vmlinux.o.

Instead, convert restore_image() and core_restore_code() to be ELF
functions.  Their code is conventional enough for objtool to be able to
understand them.

Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/974f8ceb5385e470f72e93974c70ab5c894bb0dc.1611263462.git.jpoimboe@redhat.com