gdb: LoongArch: Set the correct XML syscall filename

Now, there exists syscalls/loongarch-linux.xml, let us set the correct
XML syscall filename for LoongArch, otherwise GDB won't be able to find
the correct XML file to open and get the syscalls definitions.

It should install the package expat-devel (a library for XML parsing)
and configure --with-expat (done by default if libexpat is installed
and found at configure time) for compiling gdb in this case.

Without this patch:

(gdb) catch syscall
warning: There is no XML file to open.
warning: GDB will not be able to display syscall names nor to verify if
any provided syscall numbers are valid.
Catchpoint 1 (any syscall)

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