objtool: Get rid of reloc->list

Now that all relocs are allocated in an array, the linked list is no
longer needed.

With allyesconfig + CONFIG_DEBUG_INFO:

- Before: peak heap memory consumption: 49.02G
- After:  peak heap memory consumption: 45.56G

Link: https://lore.kernel.org/r/71e7a2c017dbc46bb497857ec97d67214f832d10.1685464332.git.jpoimboe@kernel.org


Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>