runtime,syscall,internal/poll,os: wasip1 poll_oneoff scheduler integration + net.FileListener (#5386)

* runtime,syscall,internal/poll,os: wasip1 poll_oneoff scheduler integration + net.FileListener

On wasip1 today every syscall.Read/Write blocks the entire wasm module — the
cooperative scheduler invokes poll_oneoff only for sleep/timer wakeups, and
there's no path from the net package to a working TCP server. This change
fixes both: it threads poll_oneoff through the scheduler's idle path so a
goroutine doing FD I/O parks instead of blocking the module, and it provides
enough internal/poll / os / syscall surface that upstream Go's
net.FileListener / net.FileConn works on a host-pre-opened TCP socket.

* runtime: keep scheduler_cooperative idle-wait calls direct

TestBinarySize/hifive1b/examples/echo regressed by 32 bytes after the
previous commit routed the scheduler's idle wait through a
schedulerIdleWait helper. The extra call frame + branch landed on every
non-wasip1 cooperative target, where the original direct sleepTicks /
waitForEvents calls compile to a single inlined call.

src/syscall/syscall_libc_default.go

@@ -0,0 +1,48 @@
+//go:build js || nintendoswitch || wasip2 || (wasip1 && !scheduler.tasks && !scheduler.asyncify)
+
+package syscall
+
+// These are the default Read/Write/Pread/Pwrite implementations for
+// libc-backed wasm targets that do NOT have the cooperative scheduler
+// + wasip1 netpoll integration. They are simple pass-throughs to the
+// underlying libc syscalls and block the entire wasm module if the FD
+// is in blocking mode.
+//
+// The wasip1 + cooperative-scheduler build replaces these with versions
+// that park the goroutine on EAGAIN; see syscall_libc_wasip1.go.
+
+func Write(fd int, p []byte) (n int, err error) {
+	buf, count := splitSlice(p)
+	n = libc_write(int32(fd), buf, uint(count))
+	if n < 0 {
+		err = getErrno()
+	}
+	return
+}
+
+func Read(fd int, p []byte) (n int, err error) {
+	buf, count := splitSlice(p)
+	n = libc_read(int32(fd), buf, uint(count))
+	if n < 0 {
+		err = getErrno()
+	}
+	return
+}
+
+func Pread(fd int, p []byte, offset int64) (n int, err error) {
+	buf, count := splitSlice(p)
+	n = libc_pread(int32(fd), buf, uint(count), offset)
+	if n < 0 {
+		err = getErrno()
+	}
+	return
+}
+
+func Pwrite(fd int, p []byte, offset int64) (n int, err error) {
+	buf, count := splitSlice(p)
+	n = libc_pwrite(int32(fd), buf, uint(count), offset)
+	if n < 0 {
+		err = getErrno()
+	}
+	return
+}