Use Emscripten's llvm-readobj by default

[agriyakhetarpal]

Description

This PR is a rework of pyodide/pyodide#4186. I think the premise of the original PR—which I agree with—was that it is better for us to rely on llvm-readobj from the Emscripten toolchain first, before seeking the one on PATH.

Additional info

llvm-readobj is not available with Homebrew's Emscripten recipe, but with LLVM (of course):

$(brew --prefix llvm)/bin/llvm-readobj --version

Homebrew LLVM version 19.1.6
  Optimized build.

v.s. Emscripten 3.1.58 (note the slightly older LLVM, but tip-of-tree at that time for Emscripten 3.1.58 which we use at the time of writing):

Users/agriyakhetarpal/emsdk/upstream/bin/llvm-readobj --version

LLVM (http://llvm.org/):
  LLVM version 19.0.0git
  Optimized build.

Unfortunately, I can't test what the conda-forge feedstock for Emscripten does as of now, because I can't install it on my M-series machine. I've opened an issue about it here: conda-forge/emscripten-feedstock#35

[hoodmane]

Can you make a PR to Pyodide that pulls in this commit? If CI passes on that, lgtm.

[agriyakhetarpal]

Tests are passing there, @hoodmane – thanks!