gdbserver Makefile: don't delete intermediary files

gdbserver Makefile: don't delete intermediary files

If you "make" from scratch in gdbserver/, you'll notice that make
deletes the files it considers as intermediary at the end:

  $ make clean && make
  ...
  rm i386-mmx-linux-generated.c x32-avx-avx512-linux-generated.c ...

Then, if you type make again, make will rebuild these files and rebuild
gdbserver.  To avoid this, we can add the .SECONDARY special target.  If
it has no pre-requisites, all intermediary files will be kept.

gdb/gdbserver/ChangeLog:

	* Makefile.in (.SECONDARY): Define target.