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.go

@@ -9,7 +9,6 @@ import (
 	"machine"
 	"unsafe"
 
-	"device/riscv"
 	"device/sifive"
 )
 
@@ -106,10 +105,3 @@ func sleepTicks(d timeUnit) {
 	for ticks() < target {
 	}
 }
-
-func abort() {
-	// lock up forever
-	for {
-		riscv.Asm("wfi")
-	}
-}