stm32: add minimal stm32wlex5 / lorae5 target

credit to ofauchon

src/runtime/runtime_stm32wle5.go

@@ -0,0 +1,26 @@
+//go:build stm32wle5
+// +build stm32wle5
+
+package runtime
+
+import (
+	"machine"
+)
+
+type arrtype = uint32
+
+func init() {
+	// Currently the clock is not configured, which means
+	// the MCU runs at default reset clock speed (4Mhz).
+	// Code to initialize RCC and PLL can go here.
+
+	// UART init
+	machine.Serial.Configure(machine.UARTConfig{})
+
+	// Timers init
+	initTickTimer(&machine.TIM1)
+}
+
+func putchar(c byte) {
+	machine.Serial.WriteByte(c)
+}