Add support for the X-HEEP platform

[mohasnik]

Summary

This PR adds the initial Deeploy-side infrastructure required to support C compilation for the X-HEEP target.

Changes

• Add X-HEEP as a Deeploy platform target.
• Add CMake support for selecting and configuring the X-HEEP build flow.
• Add an X-HEEP GCC toolchain configuration.
• Add an X-HEEP target library entry point under TargetLibraries.
• Wire the X-HEEP target library into Deeploy’s existing generic kernel library flow.
• Add the initial X-HEEP target package under Deeploy/Targets/xheep.
• Reuse Deeploy’s existing generic kernels where possible to avoid duplicating functionality.

Goal

The goal of this PR is to provide the Deeploy-side foundation needed to fully support compiling Deeploy-generated C code for X-HEEP.