This completely removes the hardcoded/os specific path setting for includes.
It directly queries gcc include directories.

It takes what was already done by `makefiles/libc/llvm.inc.mk`.

I replaced the `GCC_MULTI_DIR` handling by givining `CFLAGS_CPU` when
searching for include directories.
`CFLAGS` cannot be used as it will crash when `-target $(CPU_ARCH)` is
added.
It currently requires using deferred variables as `CFLAGS_CPU` can be
overwritten later in the build process.

llvm is defining NM already and `openthread` is using the symbol.

This variable can be used to specify the linker which should be used to
link C++ object files. This is needed in order to use g++ for linking
with TOOLCHAIN=llvm.

Fixes #8356

- Merge flash and flash-elf commands since they were identical except
  for the file name of the image
- Split GDB command from DBG environment variable to allow more easily
  configure front-ends for GDB via environment variables.
- Remove verbose tests of empty variables and replace by `: ${VAR:=default}`
- Remove passed command line arguments to sub-functions, they were
  unused in the functions anyway.
- Remove TUI variable, use `export DBG_EXTRA_FLAGS=-tui` to get the same
  result.