libtpool: don't clone affinity if not supported

libtpool: don't clone affinity if not supported

pthread_attr_(get/set)affinity_np() is glibc-only. This commit
disable the code path that use those functions in non-glibc
system. Fixes the following when building with musl:

libzfs.so: undefined reference to`pthread_attr_setaffinity_np'
libzfs.so: undefined reference to`pthread_attr_getaffinity_np'

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Leorize <alaviss@users.noreply.github.com>
Closes #6571