um: clean arch_ptrace() up a bit

1) take subarch-specific stuff to subarch_ptrace()
2) PTRACE_{PEEK,POKE}{TEXT,DATA} is handled by ptrace_request() just fine...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>