[gdb/testsuite] Set remotedir by default in some boards

[gdb/testsuite] Set remotedir by default in some boards

When doing a gdb_simple_compile, and downloading the resulting exec $obj
to target the result $target_obj may be a relative file path, which may give
problems when trying to do:
...
remote_exec target $target_obj
...

Fix/workaround this on some target boards by setting remotedir by default, and
add a corresponding test in gdb.testsuite/board-sanity.exp.

This doesn't work for host/target board local-remote-host-native, so xfail this.

Tested on x86_64-linux.