gdb/record: factor out reading and writing the execution log to corefile

Before this commit, the functions to read and write the execution log to
a corefile would would have a large section dedicated to handling a
single entry from the log.  This commit factors out those large sections
into their own functions, record_full_read_entry_from_bfd and
record_full_write_entry_to_bfd, respectively.

There should be no functional changes after this commit.

Reviewed-By: Christina Schimpe <christina.schimpe@intel.com>
Approved-By: Guinevere Larsen <guinevere@redhat.com>