proposal: runtime: add GOEXE

[codefromthecrypt]

Is your feature request related to a problem? Please describe.

cross-platform go code has the same concerns as the "GOEXE" variable, effectively you want to append ".exe" if windows.

Describe the solution you'd like

runtime.GOEXE which acts the same as "GOEXE" except it is a constant, so that windows binaries will know to look for .exe when launching commands.

Describe alternatives you've considered

Currently, we define moreos.EXE for this, using a constant based on the OS.

[rsc]

This seems a bit too trivial to be worth new API.

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

Based on the discussion above, this proposal seems like a likely decline.
— rsc for the proposal review group

[rsc]

No change in consensus, so declined.
— rsc for the proposal review group