powerpc: Fix clearing of the FPSCR when invoking a signal handler

As pointed out by Gary Byers, we were clearing the image of the FPSCR
(floating point status and control register) in the thread_struct before
copying it to the user stack when invoking a signal.  Thus the task
would see its FPSCR getting cleared when it took a signal.

While fixing it I noticed that our swapcontext system call was also
clearing FPSCR.  It shouldn't, so I fixed that too.

Signed-off-by: Paul Mackerras <paulus@samba.org>