gdb-cross-canadian: use correct exec-prefix path for python

gdb-cross-canadian: use correct exec-prefix path for python

Incorrect exec-prefix path was given to gdb which leads to gdb
startup failure when SDK is not installed to its original destination.
Gdb relocates the exec-prefix path, so it will work for SDKs that
are installed to different location. PYTHONHOME env in no longer
neeeded for gdb.

[YOCTO #3839]

Signed-off-by: Saul Wold <sgw@linux.intel.com>