syscall,runtime/cgo: use cgo_ldflag to link against -lsocket and -lxnet on Solaris

The -lsocket and -lxnet flags are needed for accept4 syscall on Solaris.
The runtime/cgo package doens't use them, so it doesn't make sense for
it to explicitly link against those libraries.

Instead, use the //go:cgo_ldflag in the syscall package.

Cq-Include-Trybots: luci.golang.try:gotip-solaris-amd64
Change-Id: I10db524ebf1c720a460515d8c1f362b0070bd771
Reviewed-on: https://go-review.googlesource.com/c/go/+/751760
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Quim Muntal <quimmuntal@gmail.com>

src/runtime/cgo/cgo.go

@@ -20,8 +20,6 @@ package cgo
 #cgo netbsd LDFLAGS: -lpthread
 #cgo openbsd LDFLAGS: -lpthread
 #cgo aix LDFLAGS: -Wl,-berok
-#cgo solaris LDFLAGS: -lxnet
-#cgo solaris LDFLAGS: -lsocket
 
 // Use -fno-stack-protector to avoid problems locating the
 // proper support functions. See issues #52919, #54313, #58385.

src/syscall/syscall_solaris.go

@@ -71,6 +71,9 @@ func Pipe2(p []int, flags int) error {
 	return err
 }
 
+//go:cgo_ldflag "-lxnet"
+//go:cgo_ldflag "-lsocket"
+
 //sys   accept4(s int, rsa *RawSockaddrAny, addrlen *_Socklen, flags int) (fd int, err error) = libsocket.accept4
 
 func Accept4(fd int, flags int) (int, Sockaddr, error) {