targets: simulate Circuit Playground Express in play.tinygo.org

This requires a number of changes to atsamd21 code. It's not a very
clean separation, but I couldn't come up with a better one.

src/machine/board_circuitplay_express_baremetal.go

@@ -0,0 +1,46 @@
+// +build sam,atsamd21,circuitplay_express
+
+package machine
+
+import "device/sam"
+
+// UART1 on the Circuit Playground Express.
+var (
+	UART1 = UART{
+		Buffer: NewRingBuffer(),
+		Bus:    sam.SERCOM4_USART,
+		SERCOM: 4,
+	}
+)
+
+//go:export SERCOM1_IRQHandler
+func handleUART1() {
+	defaultUART1Handler()
+}
+
+// I2C on the Circuit Playground Express.
+var (
+	// external device
+	I2C0 = I2C{
+		Bus:    sam.SERCOM5_I2CM,
+		SERCOM: 5,
+	}
+	// internal device
+	I2C1 = I2C{
+		Bus:    sam.SERCOM1_I2CM,
+		SERCOM: 1,
+	}
+)
+
+// SPI on the Circuit Playground Express.
+var (
+	SPI0 = SPI{
+		Bus:    sam.SERCOM3_SPI,
+		SERCOM: 3,
+	}
+)
+
+// I2S on the Circuit Playground Express.
+var (
+	I2S0 = I2S{Bus: sam.I2S}
+)