Add support for inline assembly

This depends on support in LLVM, which hasn't been merged yet.
See: https://reviews.llvm.org/D46437

src/device/arm/arm.go

@@ -32,6 +32,9 @@ package arm
 type __reg uint32
 type RegValue = __reg
 
+type __asm string
+func Asm(s __asm)
+
 const (
 	SCS_BASE  = 0xE000E000
 	NVIC_BASE = SCS_BASE + 0x0100

src/runtime/runtime_nrf.go

@@ -74,8 +74,8 @@ func monotime() uint64 {
 }
 
 func abort() {
-	// TODO: wfi
 	for {
+		arm.Asm("wfi")
 	}
 }