gdb: syscalls: Add some tips for LoongArch xml files

In commit a08dc2aa (gdb: syscalls: Add loongarch-linux.xml.in),
it needs special handling when generating xml file. This should at
least be mentioned in the file comment rather than git log to help
the next person who regenerates this file understand what needs to
be done, suggested by Pedro Alves, thank you.

At the beginning, I only added the tips in loongarch-linux.xml.in,
after executing the command "make" to generate loongarch-linux.xml
from loongarch-linux.xml.in, it generates the same tips in the file
loongarch-linux.xml automatically, so update loongarch-linux.xml.in
and loongarch-linux.xml together.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Approved-by: Pedro Alves <pedro@palves.net>