[gdb/testsuite] Fix gdb.server/connect-with-no-symbol-file.exp for remote target

[gdb/testsuite] Fix gdb.server/connect-with-no-symbol-file.exp for remote target

Test-case gdb.server/connect-with-no-symbol-file.exp fails with target board
remote-gdbserver-on-localhost.

The problem is here:
...
       set target_exec [gdb_remote_download target $binfile.bak $binfile]
...
A "gdb_remote_download target" copies from build to target.  So $binfile is
assumed to be a target path, but it's actually a build path.

Fix this by:
- fist copying $binfile.bak to $binfile, and
- simply doing [gdb_remote_download target $binfile].

Then, $binfile.bak is created here:
...
 # Make sure we have the original symbol file in a safe place to copy from.
 gdb_remote_download host $binfile $binfile.bak
...
and since "gdb_remote_download host" copies from build to host, $binfile.bak
is assumed to be a host path, but it's actually a build path.  This happens to
cause no problems in this configuration (because build == host), but it would
for a remote host configuration.

So let's fix this by making build rather than host the "safe place to copy
from".

Tested on x86_64-linux.