targets: rename qemu target to cortex-m-qemu

The target is intended to emulate the Cortex-M, not to be a generic QEMU
target.

src/runtime/runtime_cortexm_qemu.go

@@ -0,0 +1,45 @@
+// +build cortexm,qemu
+
+package runtime
+
+// This file implements the Stellaris LM3S6965 Cortex-M3 chip as implemented by
+// QEMU.
+
+import (
+	"device/arm"
+	"runtime/volatile"
+	"unsafe"
+)
+
+type timeUnit int64
+
+const tickMicros = 1
+
+var timestamp timeUnit
+
+//go:export Reset_Handler
+func main() {
+	preinit()
+	initAll()
+	callMain()
+	arm.SemihostingCall(arm.SemihostingReportException, arm.SemihostingApplicationExit)
+	abort()
+}
+
+const asyncScheduler = false
+
+func sleepTicks(d timeUnit) {
+	// TODO: actually sleep here for the given time.
+	timestamp += d
+}
+
+func ticks() timeUnit {
+	return timestamp
+}
+
+// UART0 output register.
+var stdoutWrite = (*volatile.Register8)(unsafe.Pointer(uintptr(0x4000c000)))
+
+func putchar(c byte) {
+	stdoutWrite.Set(uint8(c))
+}