runtime/hifive1: use CLINT peripheral for timekeeping

The CLINT is implemented both on the fe310-g002 chip and in the sifive_e
QEMU machine type. Therefore, use that peripheral for consistency.

The only difference is the clock speed, which runs at 10MHz in QEMU for
some reason instead of 32.768kHz as on the physical HiFive1 boards.

src/runtime/runtime_fe310_qemu.go

@@ -7,6 +7,8 @@ import (
 	"unsafe"
 )
 
+const tickMicros = 100 // CLINT.MTIME increments every 100ns
+
 // Special memory-mapped device to exit tests, created by SiFive.
 var testExit = (*volatile.Register32)(unsafe.Pointer(uintptr(0x100000)))
 
@@ -16,13 +18,3 @@ func abort() {
 	// Signal a successful exit.
 	testExit.Set(0x5555)
 }
-
-func ticks() timeUnit {
-	return timestamp
-}
-
-func sleepTicks(d timeUnit) {
-	// Note: QEMU doesn't seem to support the RTC peripheral at the time of
-	// writing so just simulate sleeping here.
-	timestamp += d
-}