gdbserver: fix overlap in sprintf argument and buffer

gdbserver: fix overlap in sprintf argument and buffer

While trying to build on Cygwin (gcc 10.2.0), I got:

      CXX    server.o
    /home/Baube/src/binutils-gdb/gdbserver/server.cc: In function 'void handle_general_set(char*)':
    /home/Baube/src/binutils-gdb/gdbserver/server.cc:832:12: error: 'sprintf' argument 3 overlaps destination object 'own_buf' [-Werror=restrict]
      832 |    sprintf (own_buf, "E.Unknown thread-events mode requested: %s\n",
          |    ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      833 |      mode);
          |      ~~~~~
    /home/Baube/src/binutils-gdb/gdbserver/server.cc:553:27: note: destination object referenced by 'restrict'-qualified argument 1 was declared here
      553 | handle_general_set (char *own_buf)
          |                     ~~~~~~^~~~~~~

There is indeed a problem: mode points somewhere into own_buf.  And by
the time mode gets formatted as a %s, whatever it points to has been
overwritten.  I hacked gdbserver to coerce it into that error path, and
this is the resulting message:

    (gdb) p own_buf
    $1 = 0x629000000200 "E.Unknown thread-events mode requested: ad-events mode requested: 00;10:9020fdf7ff7f0000;thread:p49388.49388;core:e;\n"

Fix it by formatting the error string in an std::string first.

gdbserver/ChangeLog:

	* server.cc (handle_general_set): Don't use sprintf with
	argument overlapping buffer.

Change-Id: I4fdf05c0117f63739413dd67ddae7bd6ee414824