riscv: implement VirtIO target

This allows running RISC-V tests in CI using QEMU, which should help
catch bugs.

src/runtime/runtime_fe310.go

@@ -16,21 +16,6 @@ import (
 
 type timeUnit int64
 
-//go:extern _sbss
-var _sbss [0]byte
-
-//go:extern _ebss
-var _ebss [0]byte
-
-//go:extern _sdata
-var _sdata [0]byte
-
-//go:extern _sidata
-var _sidata [0]byte
-
-//go:extern _edata
-var _edata [0]byte
-
 func postinit() {}
 
 //go:export main
@@ -108,24 +93,6 @@ func initPeripherals() {
 	machine.UART0.Configure(machine.UARTConfig{})
 }
 
-func preinit() {
-	// Initialize .bss: zero-initialized global variables.
-	ptr := unsafe.Pointer(&_sbss)
-	for ptr != unsafe.Pointer(&_ebss) {
-		*(*uint32)(ptr) = 0
-		ptr = unsafe.Pointer(uintptr(ptr) + 4)
-	}
-
-	// Initialize .data: global variables initialized from flash.
-	src := unsafe.Pointer(&_sidata)
-	dst := unsafe.Pointer(&_sdata)
-	for dst != unsafe.Pointer(&_edata) {
-		*(*uint32)(dst) = *(*uint32)(src)
-		dst = unsafe.Pointer(uintptr(dst) + 4)
-		src = unsafe.Pointer(uintptr(src) + 4)
-	}
-}
-
 func putchar(c byte) {
 	machine.UART0.WriteByte(c)
 }