oh…here are two defines to try forcing into your compile DB:
-D__QNXNTO__
-D__LITTLEENDIAN__
… I think the compiler might define these internally (thus the double underbar before/after) but they aren’t gleaned from extraction. It’s a long-shot, but worth a try.
since at some point you probably just want to write code, you can always consider hand-writing a clangd config file that just points at your host toolchain for everything (gcc) for forgo the qnx specific toolchain for clangd completely