Do not store the context parameter (which is used for closures and
function pointers) in the goroutine start parameter bundle for direct
functions that don't need a context parameter. This avoids storing the
(undef) context parameter and thus makes the IR to start a new goroutine
simpler in most cases.
This reduces code size in the channel.go and goroutines.go tests.
Surprisingly, all test cases (when compiled with -target=microbit) have
a changed binary, I haven't investigated why but I suppose the codegen
is slightly different for the runtime.run function (which starts the
main goroutine).
c93ddb630b