x86, x32, ptrace: Remove PTRACE_ARCH_PRCTL for x32

When I added x32 ptrace to 3.4 kernel, I also include PTRACE_ARCH_PRCTL
support for x32 GDB  For ARCH_GET_FS/GS, it takes a pointer to int64.  But
at user level, ARCH_GET_FS/GS takes a pointer to int32.  So I have to add
x32 ptrace to glibc to handle it with a temporary int64 passed to kernel and
copy it back to GDB as int32.  Roland suggested that PTRACE_ARCH_PRCTL
is obsolete and x32 GDB should use fs_base and gs_base fields of
user_regs_struct instead.

Accordingly, remove PTRACE_ARCH_PRCTL completely from the x32 code to
avoid possible memory overrun when pointer to int32 is passed to
kernel.

Link: http://lkml.kernel.org/r/CAMe9rOpDzHfS7NH7m1vmD9QRw8SSj4Sc%2BaNOgcWm_WJME2eRsQ@mail.gmail.com
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: <stable@vger.kernel.org> v3.4

arch/x86/kernel/ptrace.c

@@ -1211,12 +1211,6 @@ static long x32_arch_ptrace(struct task_struct *child,
 					     0, sizeof(struct user_i387_struct),
 					     datap);
 
-		/* normal 64bit interface to access TLS data.
-		   Works just like arch_prctl, except that the arguments
-		   are reversed. */
-	case PTRACE_ARCH_PRCTL:
-		return do_arch_prctl(child, data, addr);
-
 	default:
 		return compat_ptrace_request(child, request, addr, data);
 	}