gcc: disable libsanitizer for uclibc

libsanitizer requires header files which are not present in uclibc.

Also, libsanitizer is disabled with uclibc for buildroot[1] and
openwrt[2].

[1]: https://git.busybox.net/buildroot/commit/?id=554e29e267e6b36a0fd78c82cbad2c82d939eb7f
[2]: https://gitlab.labs.nic.cz/turris/openwrt/commit/5f372a8ca08de4b2253b87253ca7762450e5641f