gdb/testsuite: avoid creating files in gdb/testsuite directory

gdb/testsuite: avoid creating files in gdb/testsuite directory

I spotted that the test gdb.dwarf2/dw2-using-debug-str.exp was
creating an output file called debug_str_section in the root
build/gdb/testsuite directory instead of using the
build/gdb/testsuite/output/gdb.dwarf2/dw2-using-debug-str/ directory.

This appears to be caused by a missing '$' character.  We setup a
variable debug_str_section which contains a path within the output
directory, but then when we build the objcopy command we use
'debug_str_section' without a '$' prefix, as a result, we create the
debug_str_section file.

This commit adds the missing '$', the file is now created in the
output directory.