targets: add hifive1-qemu for testing RISC-V bare metal in QEMU

Most tests don't pass yet, so can't add this test to the standard tests,
yet.

src/runtime/runtime_fe310_qemu.go

@@ -0,0 +1,16 @@
+// +build fe310,qemu
+
+package runtime
+
+import (
+	"runtime/volatile"
+	"unsafe"
+)
+
+// Special memory-mapped device to exit tests, created by SiFive.
+var testExit = (*volatile.Register32)(unsafe.Pointer(uintptr(0x100000)))
+
+func abort() {
+	// Signal a successful exit.
+	testExit.Set(0x5555)
+}