[gdb/testsuite] Fix gdb.dwarf2/fission-relative-dwo.exp with cc-with-gdb-index

[gdb/testsuite] Fix gdb.dwarf2/fission-relative-dwo.exp with cc-with-gdb-index

When running with target board cc-with-gdb-index, we run into:
...
gdb compile failed, warning: Could not find DWO CU \
  fission-relative-dwo.dwo(0x1234) referenced by CU at offset 0xc7 \
  [in module outputs/gdb.dwarf2/fission-relative-dwo/.tmp/fission-relative-dwo]
UNTESTED: gdb.dwarf2/fission-relative-dwo.exp: fission-relative-dwo.exp
ERROR: failed to compile fission-relative-dwo
...

The problem is that:
- the .dwo file is found relative to the executable, and
- cc-with-tweaks.sh moves the executable to a temp dir, but not
  the .dwo file.

Fix this by copying the .dwo file alongside the executable in the temp dir.

Verified changes using shellcheck.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-08-02  Tom de Vries  <tdevries@suse.de>

	* contrib/cc-with-tweaks.sh: Copy .dwo files alongside executable.