Allow MacOS-based compilation using official espressif crosstool-NG toolchain

Missing feature

NodeMCU firmware can't be built on MacOS machines.

Justification

https://github.com/espressif/crosstool-NG already exists and, being crosstool-based, centralises host platform differences so that nodemcu-firmware can concentrate on NodeMCU and not, say, differences between MacOS and Linux.

#3687 is a comparable request for building using espressif-supplied toolchains but on Windows. At a glance, it seems to include some work that I have also done myself (e.g. accommodating the possibility that BYTE_ORDER might already be defined).

Workarounds

I have a clone of nodemcu-firmware which I have tweaked so that I can build on MacOS 15.5 on an Apple Silicon machine.