sh: kgdb: do not reload VBR while handling debugger breackpoint

Save the VBR allowing GDB to dump full registers set but do not reload it
as soon as the kgdb_handle_exception is invoked.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>