Revert "runtime: map; llgo/ssa: MapUpdate"

internal/runtime/z_rt.go

@@ -93,3 +93,10 @@ func stringTracef(fp c.FilePtr, format *c.Char, s String) {
 }
 
 // -----------------------------------------------------------------------------
+
+// Zeroinit initializes memory to zero.
+func Zeroinit(p unsafe.Pointer, size uintptr) unsafe.Pointer {
+	return c.Memset(p, 0, size)
+}
+
+// -----------------------------------------------------------------------------