Blackfin: fix command line corruption with DEBUG_DOUBLEFAULT

Commit 6b3087c6 (which introduced Blackfin SMP) broke command line passing
when the DEBUG_DOUBLEFAULT config option was enabled.  Switch the code to
using a scratch register and not R7 which holds the command line.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>