syscall: remove linknames to runtime symbols for aix/ppc64

Replaces //go:linkname by assembly functions for syscall
functions on aix/ppc64.
Since the new runtime internal ABI, this was triggering an error if
syscall.Syscall6 was called by others packages like x/sys/unix.
This commit should remove every future occurences of this problem.

Fixes #28769

Change-Id: I6a4bf77472ee1e974bdb76b27e74275e568f5a76
Reviewed-on: https://go-review.googlesource.com/c/153997
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

src/runtime/syscall_aix.go

@@ -57,7 +57,6 @@ var (
 // Syscall is needed because some packages (like net) need it too.
 // The best way is to return EINVAL and let Golang handles its failure
 // If the syscall can't fail, this function can redirect it to a real syscall.
-//go:linkname syscall_Syscall syscall.Syscall
 //go:nosplit
 func syscall_Syscall(fn, a1, a2, a3 uintptr) (r1, r2, err uintptr) {
 	return 0, 0, _EINVAL
@@ -65,12 +64,10 @@ func syscall_Syscall(fn, a1, a2, a3 uintptr) (r1, r2, err uintptr) {
 
 // This is syscall.RawSyscall, it exists to satisfy some build dependency,
 // but it doesn't work.
-//go:linkname syscall_RawSyscall syscall.RawSyscall
 func syscall_RawSyscall(trap, a1, a2, a3 uintptr) (r1, r2, err uintptr) {
 	panic("RawSyscall not available on AIX")
 }
 
-//go:linkname syscall_syscall6 syscall.syscall6
 //go:nosplit
 func syscall_syscall6(fn, nargs, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr) {
 	c := getg().m.libcall
@@ -84,7 +81,6 @@ func syscall_syscall6(fn, nargs, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err ui
 	return c.r1, 0, c.err
 }
 
-//go:linkname syscall_rawSyscall6 syscall.rawSyscall6
 //go:nosplit
 func syscall_rawSyscall6(fn, nargs, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr) {
 	c := getg().m.libcall

src/syscall/asm_aix_ppc64.s

+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "textflag.h"
+
+//
+// System calls for aix/ppc64 are implemented in ../runtime/syscall_aix.go
+//
+
+TEXT ·syscall6(SB),NOSPLIT,$0
+	JMP	runtime·syscall_syscall6(SB)
+
+TEXT ·rawSyscall6(SB),NOSPLIT,$0
+	JMP	runtime·syscall_rawSyscall6(SB)
+
+TEXT ·RawSyscall(SB),NOSPLIT,$0
+	JMP	runtime·syscall_RawSyscall(SB)
+
+TEXT ·Syscall(SB),NOSPLIT,$0
+	JMP	runtime·syscall_Syscall(SB)