scripts: Port Linux's gen_compile_commands.py to U-Boot

This script generates a database of compiler flags, namely
compile_commands.json. It is quite useful for text editors that use
clangd LSP (e.g. Vim, Neovim).

It was ported from Linux's sources:
- tag: v6.4
- revision 6995e2de6891c724bfeb2db33d7b87775f913ad1

Modifications for U-Boot compatibility will be added in a follow-up
commit.

Signed-off-by: Joao Marcos Costa <jmcosta944@gmail.com>
Tested-by: Joao Paulo Goncalves <joao.goncalves@toradex.com>