compiler: implement nil checks

This commit implements nil checks for all platforms. These nil checks
can be optimized on systems with a MMU, but since a major target is
systems without MMU, keep it this way for now.

It implements three checks:
  * Nil checks before dereferencing a pointer.
  * Nil checks before calculating an address (*ssa.FieldAddr and
    *ssa.IndexAddr)
  * Nil checks before calling a function pointer.

The first check has by far the biggest impact, with around 5% increase
in code size. The other checks only trigger in only some test cases and
have a minimal impact on code size.
This first nil check is also the one that is easiest to avoid on systems
with MMU, if necessary.

src/runtime/panic.go

@@ -27,6 +27,11 @@ func _recover() interface{} {
 	return nil
 }
 
+// Panic when trying to dereference a nil pointer.
+func nilpanic() {
+	runtimePanic("nil pointer dereference")
+}
+
 // Check for bounds in *ssa.Index, *ssa.IndexAddr and *ssa.Lookup.
 func lookupBoundsCheck(length uintptr, index int) {
 	if index < 0 || index >= int(length) {