system: fix return value of gc() and breakpoint()
system: fix return value of gc() and breakpoint()

These need to return void (undefined), and not themselves (which
is what happens if SET_RVAL() is not called, due to spidermonkey
reusing the slot for arguments.callee)

endlessm/gjs,endlessm/gjs,endlessm/gjs,endlessm/gjs,endlessm/gjs