Make native-only loaders optional on Emscripten

[zardus]

Summary

• exclude pyxdia and uefi-firmware from Emscripten dependency resolution
• make PE PDB support degrade cleanly when pyxdia is unavailable
• register the UEFI backend only when its native-only dependency is importable

The remaining CLE loaders, including ELF, stay available in the browser runtime.

Validation

Loaded and analyzed the AMD64 fauxware ELF under Pyodide. Native PE and UEFI imports continue to work with their dependencies installed.

[zardus]

Tandem PRs — please test these together:

• Support wasm32 hosts in VEX basic types vex#82
• Add wasm32 Pyodide wheel support pyvex#555
• Select the Pyodide Z3 build on Emscripten claripy#737
• Make native-only loaders optional on Emscripten #702
• Guard optional UEFI dispatch registration #703
• Test CLE under Pyodide #704
• Add Pyodide WebAssembly wheel support pypcode#283
• Add first-class WebAssembly runtime support angr#6658

[angr-bot]

Corpus decompilation diffs can be found at angr/dec-snapshots@master...angr/cle_702