[gdb/testsuite] Fix quoting issues in gdb.dwarf2 for remote host

A few test-cases in gdb.dwarf2 use something like:
...
  additional_flags=\"-DFOO=BAR + 10\"
...
which doesn't work on remote host.

Fix this by introducing a new proc quote_for_host that also works for remote
host, such that we have:
...
  additional_flags=[quote_for_host -DFOO=BAR + 10]
...

Tested on x86_64-linux.