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.
1802e775ad