platforms should be able to provide a _start prototype

[psifertex]

required for #3925 (among other improvements)
Would also allow specifying WinMain, eg

[plafosse]

This feature would allow the specification of WinMain's parameters but would not aid in the discovery of WinMain in the first place. For that we need a function signature matcher on the windows CRT.

[plafosse]

Added in 3.5.4333