usb/cdc: fix RP2 USB CDC TX race with cores scheduler

[rdon-key]

Fixes #5377.

This PR fixes a USB CDC TX race on RP2 targets when using -scheduler=cores.

The issue was reproduced on Raspberry Pi Pico / RP2040 with TinyGo 0.41.1.
RP2350 may be affected as well because it shares the RP2 USB backend and the
same USB CDC TX state machine.

Cause

USB CDC TX is driven from two places:

• Write() via kickTx()
• the USB TX completion handler via txhandler()

The previous code used inflight both as:

• the number of bytes currently submitted to the USB IN endpoint
• an implicit guard for whether the TX pump is active

That is not sufficient on multicore targets.

With -scheduler=cores, Write() may run on one core while the USB TX
completion handler runs on another core. In that case, kickTx() can observe the
TX path as idle while txhandler() is still processing a completed packet and
chaining the next one.

That can allow concurrent or re-entrant calls into sendFromRing() /
machine.SendUSBInPacket() for the same USB IN endpoint, which can corrupt USB
CDC output.

Fix

This PR adds a separate atomic txActive flag.

txActive represents ownership of the USB CDC TX pump. inflight remains only
the number of bytes currently submitted to the endpoint.

kickTx() now starts the TX pump only if it can acquire txActive with CAS.
The TX completion handler keeps TX pump ownership while chaining packets, and
ownership is released only when the TX ring is empty.

A final ring re-check is used when releasing ownership to avoid a missed wakeup
if Write() appends data while txActive is still set.

Manual test

Test program:

package main

import "time"

func main() {
	time.Sleep(2 * time.Second)

	println("start usb cdc tx stress")

	for i := 0; i < 1000; i++ {
		println("usb cdc tx test:", i, "abcdefghijklmnopqrstuvwxyz0123456789")
		time.Sleep(1 * time.Millisecond)
	}

	println("test finished")
}

Commands:

~/work/tinygo/build/tinygo flash -target=pico -scheduler=cores -monitor usbcdc_tx_stress.go
~/work/tinygo/build/tinygo flash -target=pico -scheduler=tasks -monitor usbcdc_tx_stress.go

Results:

Before this fix:
- TinyGo 0.41.1 + -scheduler=cores: USB CDC output was corrupted
- TinyGo 0.41.1 + -scheduler=tasks: OK

After this fix:
- local dev build + -scheduler=cores: OK
- local dev build + -scheduler=tasks: OK

The -scheduler=cores test was repeated multiple times without observing
dropped or corrupted USB CDC output.