gdb/record: Define new version of the record-save section

gdb/record: Define new version of the record-save section

With the changes to the internal representation of the history, we can
no longer support the previous record save format. This commit makes it
official, documenting the new format and changing the magic number.

Reviewed-By: Christina Schimpe <christina.schimpe@intel.com>
Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Guinevere Larsen <guinevere@redhat.com>