Add support for ENABLE_OVERRIDE

To enable rpmalloc's overriding of C malloc/free functions, this enables rpmalloc to be used instead of the system allocator for C/C++ code dependencies as well, not just for Rust code.

Easy to enable (and should be a feature flag on the crates) but do have to make sure it links properly on all platforms and targets.

Start by locating the crate feature flags and the existing ENABLE_OVERRIDE handling, then trace how the allocator is linked for each supported platform and target. Confirm the feature can be enabled for dependent C/C++ code and that builds and linking succeed across the relevant targets.

Written by the indexing model from the issue text.