sparc64: fix toolchain building when C++ is enabled

Disable libsanitizer for sparc64, too. Same problem as for
sparc, see https://bugs.busybox.net/show_bug.cgi?id=7951



Reported-By: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>