runtime/interrupt: add Checkpoint type

This type can be used to jump back to a previous position in a program
from inside an interrupt. This is useful for baremetal systems that
implement wfi but not wfe, and therefore have no easy (race-free) way to
wait until a flag gets changed inside an interrupt. This is an issue on
RISC-V, where this is racy (the interrupt might happen after the check
but before the wfi instruction):

    configureInterrupt()
    for flag.Load() != 0 {
        riscv.Asm("wfi")
    }

compiler/compiler.go

@@ -1895,6 +1895,8 @@ func (b *builder) createFunctionCall(instr *ssa.CallCommon) (llvm.Value, error)
 		case name == "runtime.exportedFuncPtr":
 			_, ptr := b.getFunction(instr.Args[0].(*ssa.Function))
 			return b.CreatePtrToInt(ptr, b.uintptrType, ""), nil
+		case name == "(*runtime/interrupt.Checkpoint).Save":
+			return b.createInterruptCheckpoint(instr.Args[0]), nil
 		case name == "internal/abi.FuncPCABI0":
 			retval := b.createDarwinFuncPCABI0Call(instr)
 			if !retval.IsNil() {