sparc64: Reschedule KGDB capture to a software interrupt.

[ Upstream commit 42cc77c861e8e850e86252bb5b1e12e006261973 ]

Otherwise it might interrupt switch_to() midstream and use
half-cooked register window state.

Reported-by: Chris Torek <chris.torek@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>