classes, conf: Exclude native and nativesdk recipes from libc specific override

classes, conf: Exclude native and nativesdk recipes from libc specific override

Based on suggestions from RP

libc-uclibc and libc-glibc overrides are for denoting
system C library in use on the target and not for the
host therefore we make sure that the override only takes
effect for target recipes only.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>