gendwarfksyms: use preferred form of sizeof for allocation

The preferred form is to use the variable being allocated to, rather
than explicitly supplying a type name which might become stale.

Also do this for memset.

Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Giuliano Procida <gprocida@google.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>