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")
    }

src/runtime/asm_riscv.S

@@ -50,3 +50,12 @@ tinygo_longjmp:
     lw sp, 0(a0)       // jumpSP
     lw a1, REGSIZE(a0) // jumpPC
     jr a1
+
+.section .text.tinygo_checkpointJump
+.global tinygo_checkpointJump
+tinygo_checkpointJump:
+    // Note: the code we jump to assumes a0 is non-zero, which is already the
+    // case because that's the stack pointer.
+    mv sp, a0          // jumpSP
+    csrw mepc, a1      // update MEPC value, so we resume there after the mret
+    mret               // jump to jumpPC