scripts/kallsyms: change table to store (strcut sym_entry *)

The symbol table is extended every 10000 addition by using realloc(),
where data copy might occur to the new buffer.

To decrease the amount of possible data copy, let's change the table
to store the pointer.

The symbol type + symbol name part is appended at the end of
(struct sym_entry), and allocated together with the struct body.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>