Update the NetBSD system call table to match NetBSD-current.

Generated from sys/sys/syscall.h revision 1.319.

We can safely remove the _lwp_gettid syscall, which was never exposed
in libc and never made it into a release.

gdb/ChangeLog:

2021-07-23  Frederic Cambus  <fred@statdns.com>

	* syscalls/netbsd.xml: Regenerate.