alpha: convert to generic sys_ptrace

This patch converts alpha to the generic sys_ptrace.  We use
force_successful_syscall_return to avoid having to pass the pt_regs pointer
down to the function.  I think the removal of the assemly stub is correct,
but I could only compile-test this patch, so please give it a spin before
commiting :)

Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>