Contribution description

Hi! 🦎

When using llvm and address sanitation, the XFA trip the sanitizer.
This PR attempts to fix this by adding the no_sanitize attribute to the XFA macros. Sadly, this attribute is not known by gnu, a guard is hence needed. I'm open for alternatives as I dislike this solution but it is the best I could come up with.

Testing procedure

Before this patch:

Go to examples/gnrc_minimal and run TOOLCHAIN=llvm make all-asan and then make term.
You should see an error similar to this:

==3374719==ERROR: AddressSanitizer: global-buffer-overflow on address 0x080774e0 at pc 0x0804af5e bp 0x0808eb88 sp 0x0808eb78
READ of size 4 at 0x080774e0 thread T0
    #0 0x804af5d in _auto_init_module /RIOT/sys/auto_init/auto_init.c:40
    #1 0x804af5d in auto_init /RIOT/sys/auto_init/auto_init.c:339
    #2 0x804b375 in main_trampoline /RIOT/core/lib/init.c:56
    #3 0xf76bc7b8 in makecontext (/lib32/libc.so.6+0x4a7b8)
...

After applying this PR, the example can be build and run with llvm or gcc, with or without asan.