std: More type checks for Thread startFn return type
std: More type checks for Thread startFn return type

Closes #4756

raulgrell/zig,andrewrk/zig,zig-lang/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,andrewrk/zig