avr: convert initialization from asm to Go

This increases code size by 1 instruction (2 bytes) because LLVM isn't
yet smart enough to recognize that it doesn't need to clear a register
to use 0: it can just use r1 which is always 0 according to the
convention. It makes initialization a lot easier to read, however.

src/runtime/runtime_avr.go

@@ -4,6 +4,7 @@ package runtime
 
 import (
 	"device/avr"
+	"unsafe"
 )
 
 const BOARD = "arduino"
@@ -28,6 +29,25 @@ const (
 	WDT_PERIOD_2S
 )
 
+var (
+	_extern__sbss unsafe.Pointer // defined by the linker
+	_extern__ebss unsafe.Pointer // defined by the linker
+)
+
+func preinit() {
+	// Initialize .bss: zero-initialized global variables.
+	ptr := uintptr(unsafe.Pointer(&_extern__sbss))
+	for ptr != uintptr(unsafe.Pointer(&_extern__ebss)) {
+		*(*uint8)(unsafe.Pointer(ptr)) = 0
+		ptr += 1
+	}
+}
+
+func postinit() {
+	// Enable interrupts after initialization.
+	avr.Asm("sei")
+}
+
 func init() {
 	initUART()
 }