gdb: syscalls: Generate loongarch-linux.xml

Make use of the command "make" to generate loongarch-linux.xml
from loongarch-linux.xml.in.

Like this:

  $ git clone https://sourceware.org/git/binutils-gdb.git gdb.git
  $ cd gdb.git/gdb/syscalls/
  $ make

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