Obtain the `heap_base` from Rust

Discussed in https://github.com/0xMiden/compiler/pull/604#issuecomment-3089661935

The `wasm_ld` exports `__heap_base` that holds the address for the start of the heap.

https://surma.dev/things/c-to-webassembly/

https://stackoverflow.com/questions/69994454/accessing-heap-base-when-building-a-wasm-binary-with-rust

Here are some allocators that use `__heap_base` for blockchains that use Wasm - https://github.com/search?q=static+__heap_base%3A+language%3ARust&type=code&l=Rust