Added ghidra build script

[nimashoghi]

I was told on the Compiler Explorer Discord that I should post this here.

This script basically does the following:

1. Installs all dependencies needed for Ghidra (binutils-dev and zlib1g-dev)
2. Clones my fork of the Ghidra repository, which fixes a runtime error I had when using Ghidra's decompiler with Compiler Explorer.
3. Builds the Ghidra decompiler tooling and the Ghidra SLEIGH compiler.
4. Uses the Ghidra SLEIGH compiler to generate architecture specification files for x86-64 and x86.
5. Moves these specification files to /usr/local/share/ghidra/, so it can be used by the Ghidra compiler explorer tooling.

See this PR for more information.

[AbrilRBS]

If you see how it's done for other compilers, they skip the build if the existing version is already the latest one (https://github.com/compiler-explorer/misc-builder/blob/main/build/build-rustc-cg-gcc.sh#L30) Could you also implement this?

[mattgodbolt]

Catching up here: can you respond to @RubenRBS's comment here - it's important for us not to waste resources building unchanged code over and over again. If that's not clear, let me know and I can try picking this up.