llvm.inc.mk: completely replace detection of GCC includes
llvm.inc.mk: completely replace detection of GCC includes

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.

yogo1212/RIOT,basilfx/RIOT,kaspar030/RIOT,authmillenon/RIOT,BytesGalore/RIOT,gebart/RIOT,josephnoir/RIOT,aeneby/RIOT,rfuentess/RIOT,smlng/RIOT,authmillenon/RIOT,jasonatran/RIOT,cladmi/RIOT,aeneby/RIOT,aeneby/RIOT,rfuentess/RIOT,toonst/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,x3ro/RIOT,RIOT-OS/RIOT,cladmi/RIOT,A-Paul/RIOT,BytesGalore/RIOT,biboc/RIOT,basilfx/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,cladmi/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,smlng/RIOT,x3ro/RIOT,ant9000/RIOT,ant9000/RIOT,yogo1212/RIOT,mtausig/RIOT,smlng/RIOT,smlng/RIOT,rfuentess/RIOT,BytesGalore/RIOT,rfuentess/RIOT,kYc0o/RIOT,authmillenon/RIOT,kYc0o/RIOT,toonst/RIOT,jasonatran/RIOT,lazytech-org/RIOT,miri64/RIOT,josephnoir/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,toonst/RIOT,BytesGalore/RIOT,yogo1212/RIOT,lazytech-org/RIOT,authmillenon/RIOT,lazytech-org/RIOT,miri64/RIOT,mtausig/RIOT,miri64/RIOT,ant9000/RIOT,biboc/RIOT,yogo1212/RIOT,aeneby/RIOT,josephnoir/RIOT,OTAkeys/RIOT,basilfx/RIOT,kaspar030/RIOT,OlegHahm/RIOT,mfrey/RIOT,A-Paul/RIOT,basilfx/RIOT,rfuentess/RIOT,basilfx/RIOT,kaspar030/RIOT,gebart/RIOT,smlng/RIOT,x3ro/RIOT,josephnoir/RIOT,mtausig/RIOT,mfrey/RIOT,authmillenon/RIOT,biboc/RIOT,miri64/RIOT,mfrey/RIOT,mfrey/RIOT,jasonatran/RIOT,mtausig/RIOT,RIOT-OS/RIOT,miri64/RIOT,cladmi/RIOT,A-Paul/RIOT,x3ro/RIOT,kYc0o/RIOT,jasonatran/RIOT,authmillenon/RIOT,BytesGalore/RIOT,toonst/RIOT,biboc/RIOT,ant9000/RIOT,cladmi/RIOT,gebart/RIOT,gebart/RIOT,A-Paul/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kaspar030/RIOT,A-Paul/RIOT,kYc0o/RIOT,lazytech-org/RIOT,gebart/RIOT,lazytech-org/RIOT,mfrey/RIOT,x3ro/RIOT,yogo1212/RIOT,mtausig/RIOT,josephnoir/RIOT,kYc0o/RIOT,toonst/RIOT,aeneby/RIOT,kaspar030/RIOT,OlegHahm/RIOT,biboc/RIOT,ant9000/RIOT