[gdb/testsuite] Normalize gdbserver path name

Currently for the target board remote-gdbserver-on-localhost we use the
gdbserver file on build, using a file name which includes "/../".

Fix this by using a normalized file name instead.

This allows us to be more restrictive about which files REMOTE_TARGET_USERNAME
can access:
...
-    remote_exec build "chmod go-rx $objdir/outputs"
+    remote_exec build "chmod go-rx $objdir"
...

Tested on x86_64-linux.