target/hppa: Avoid accessing %gr0 when raising exception

target/hppa: Avoid accessing %gr0 when raising exception

The value of unwind_breg may reference register %r0, but we need to avoid
accessing gr0 directly and use the value 0 instead.

At runtime I've seen unwind_breg being zero with the Linux kernel when
rfi is used to jump to smp_callin().

Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Bruno Haible <bruno@clisp.org>