crash: add VMCOREINFO macro to define offset in a struct declared by typedef

The existing macro VMCOREINFO_OFFSET() can't be used for structures
declared via typedef because "struct" is not part of type definition.

Create another macro for this purpose.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Baoquan He <bhe@redhat.com>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20200709132344.760-2-john.ogness@linutronix.de