Auto merge of #1552 - gsquire:add-pthread_attr_get_np, r=gnzlbg
Auto merge of #1552 - gsquire:add-pthread_attr_get_np, r=gnzlbg

NetBSD pthread_attr_get_np

This patch adds a signature for the [`pthread_attr_get_np`](https://netbsd.gw.com/cgi-bin/man-cgi?pthread_attr_get_np+3+NetBSD-current) function for all architectures on NetBSD.

Closes #1538

rust-lang/libc,rust-lang/libc,rust-lang/libc