Simplify the inthand_t type for x86.

[kib]

Summary

Types for the x86 interrupt handler entry points cannot be expressed in C. For i386, the existing type is some approximation, lacking __stdcall attribute and still being wrong for the return type. For amd64, it cannot be even approximated.

Instead, use void (*)(void) which equally serves the purpose, is simpler and allows to unify i386 and amd64.

[jhb]

Fine with me. The things can't be written directly in C anyway so trying to have a fancy prototype is rather pointless.