feat: add instrument_hooks_callgrind_add_obj_skip

Expose CALLGRIND_ADD_OBJ_SKIP through the C API so language integrations
(pytest-codspeed, codspeed-rust, ...) can register --obj-skip paths at
runtime without re-implementing the userreq trapdoor.

The helper also resolves the realpath and registers it when it differs:
callgrind matches obj-skip entries via exact strcmp against the
loader-resolved name in obj_node, so a symlink-only registration would
silently miss.

Refs COD-2658.

Author: not-matthias