asan: Really don't intercept memset in arch/x86/boot

This PR fixes the following link errors with make oldconfig based on Debian's 3.15 kernel:
...
arch/x86/boot/compressed/eboot.o: In function `setup_graphics':
eboot.c:(.text+0x136c): undefined reference to `asan_memset'
arch/x86/boot/compressed/eboot.o: In function `make_boot_params':
eboot.c:(.text+0x2149): undefined reference to `asan_memset'
...

It would be really nice to upstream kasan to GCC and to Linux even if it is not perfect yet.