gdb: apply post-patch hook on both the host and target builds

When a Xtensa specific patch needs to be applied, it should be applied
both when doing the target and the host build.

This change is part of the effort to remove the differences between
host and target post-extract and post-patch hooks, that are
problematic for the out-of-tree support.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>