[gccrs] Adding Infrastructure to Compile the Rust 'alloc' Crate

This project implements the compiler infrastructure—including missing intrinsics and lang items—required to compile Rust’s alloc crate in GCC Rust (gccrs). The work focuses on bridging architectural gaps and ensuring alloc is ready for integration once core support is finalized. Deliverables include core heap-related functionality, regression tests, and documentation of the resolved compiler limitations.