runtime: remove __go_ptrace on AIX

AIX ptrace syscalls doesn't have the same semantic than the glibc one.
The syscall package is already handling it correctly so disable the new
__go_ptrace C function for AIX.

Change-Id: I5a2abe0dceaf2726ec0f6ad240869a630ab6727b
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/256777
Trust: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

libgo/runtime/go-varargs.c

diff --git a/libgo/runtime/go-varargs.c b/libgo/runtime/go-varargs.c
index f9270a9..9cb4a7e 100644
--- a/libgo/runtime/go-varargs.c
+++ b/libgo/runtime/go-varargs.c
@@ -114,7 +114,9 @@
 
 #endif
 
-#ifdef HAVE_SYS_PTRACE_H
+// AIX ptrace is really different from Linux ptrace. Let syscall
+// package handles it.
+#if defined(HAVE_SYS_PTRACE_H) && !defined(_AIX)
 
 // Despite documented appearances, this is actually implemented as
 // a variadic function within glibc.