Create compile time flag for disabling backtrace support

[frank-emrich]

Following #98 and #99, there is a certain runtime overhead incurred at every stack switching operation, due to moving data between StackLimit objects and the VMRuntimeLimits. In addition, the StackLimit objects in each ContinuationObject occupy space. This is in part due to our usage of libcalls for switching stacks and should therefore be avoidable in the future.

We should therefore have a flag to disable this particular implementation of backtrace support. This is not entirely trivial, as the stack_limit value inside StackLimits objects will be used to enabled stack overflow checks, which would be disabled by that new flag.