sim/erc32: Use int32_t as event callback argument

sim/erc32: Use int32_t as event callback argument

Clang generates a warning if an argument is passed to a function without
prototype (zero arguments, even without (void)).  Such calls are deprecated
forms of indefinite arguments passing ("-Wdeprecated-non-prototype").
On the default configuration, it causes a build failure (unless
"--disable-werror" is specified).

To fix that, this commit makes struct evcell to use int32_t as a callback
(cfunc) argument of an event.  int32_t is chosen because "event" function
accepts "int32_t arg".