package/gcc: disable libsanitizer for uClibc

libsanitizer requires wordexp() support which we lack in our current
default uClibc configurations (and it's fat & big).
Hence disable it when the toolchain is uClibc-based.
It only affects gcc 4.9+ since it's default on now for supported
platforms.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>