runtime: move epoll syscalls to runtime/internal/syscall

This change moves Linux epoll's syscalls implementation to the
"runtime/internal/syscall" package. The intention in this CL was to
minimise behavioural changes but make the code more generalised. This
also will allow adding new syscalls (like epoll_pwait2) without the
need to implement assembly stubs for each arch.

It also drops epoll_create as not all architectures provide this call.
epoll_create1 was added to the kernel in version 2.6.27 and Go requires
Linux kernel version 2.6.32 or later since Go 1.18. So it is safe to
always use epoll_create1.

This is a resubmit as the previous CL 421994 was reverted due to test
failures after the merge with the master. The issue was fixed in
CL 438615

For #53824
For #51087

Change-Id: I1bd0f23a85b4f9b80178c5dd36fd3e95ff4f9648
Reviewed-on: https://go-review.googlesource.com/c/go/+/440115
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>

src/runtime/internal/syscall/defs_linux_mips64x.go

@@ -0,0 +1,32 @@
+// Copyright 2022 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.
+
+//go:build linux && (mips64 || mips64le)
+
+package syscall
+
+const (
+	SYS_FCNTL         = 5070
+	SYS_EPOLL_CTL     = 5208
+	SYS_EPOLL_PWAIT   = 5272
+	SYS_EPOLL_CREATE1 = 5285
+	SYS_EPOLL_PWAIT2  = 5441
+
+	EPOLLIN       = 0x1
+	EPOLLOUT      = 0x4
+	EPOLLERR      = 0x8
+	EPOLLHUP      = 0x10
+	EPOLLRDHUP    = 0x2000
+	EPOLLET       = 0x80000000
+	EPOLL_CLOEXEC = 0x80000
+	EPOLL_CTL_ADD = 0x1
+	EPOLL_CTL_DEL = 0x2
+	EPOLL_CTL_MOD = 0x3
+)
+
+type EpollEvent struct {
+	Events    uint32
+	pad_cgo_0 [4]byte
+	Data      [8]byte // unaligned uintptr
+}