gdb: syscalls: Add loongarch case in update-linux-from-src.sh

It shows that "Don't know how to generate loongarch-linux.xml.in"
when using the script update-linux-from-src.sh to regenerate the
syscall group info against Linux kernel, just add loongarch case.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Approved-By: John Baldwin <jhb@FreeBSD.org>