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/defs_linux_386.go

@@ -93,17 +93,6 @@ const (
 	_O_NONBLOCK = 0x800
 	_O_CLOEXEC  = 0x80000
 
-	_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
-
 	_AF_UNIX    = 0x1
 	_SOCK_DGRAM = 0x2
 )
@@ -254,11 +243,6 @@ type sigevent struct {
 	_ [_sigev_max_size - unsafe.Sizeof(sigeventFields{})]byte
 }
 
-type epollevent struct {
-	events uint32
-	data   [8]byte // to match amd64
-}
-
 type sockaddr_un struct {
 	family uint16
 	path   [108]byte