[gdb/testsuite] Set remotedir in local-remote-host-native.exp

[gdb/testsuite] Set remotedir in local-remote-host-native.exp

In commit ff581559f9d ("[gdb/testsuite] Add gdb.testsuite/board-sanity.exp") I
removed handling of HOST_DIR in local-remote-host-native.exp to fix FAILs
in test-case gdb.testsuite/board-sanity.exp.

Reintroduce handling of HOST_DIR using remotedir, now that using remotedir for
a host board no longer make compilation fail due to commit 80d6c79866f
("[gdb/testsuite] Handle remotedir in remote_upload").

This fixes an XFAIL in gdb.testsuite/board-sanity.exp, introduced in commit
3741934fdb0 ("[gdb/testsuite] Set remotedir by default in some boards").

Tested on x86_64-linux.