4.0 patch: Implement gij -Xss option

[Bryce McKinlay]

This is a simple subset of a larger patch I was working on to change the 
default stack size and implement stack overflow detection. 
Unfortunately, it turns out that there are currently some CG issues that 
prevent stack overflow detection from working robustly. Specifically, 
because the GC can run on any thread, and itself requires some stack, 
there must be plenty of stack available for the signal handler to 
execute as the signal handler needs to allocate. One solution might be 
to ensure that the GC always runs on its own thread(s) which are 
guaranteed to have plenty of stack space.

Limitations of this patch:

- There is no support for detecting stack overflow and throwing 
StackOverflowException. If a stack overflows, it will bring down the 
whole runtime (this is the same as what happens currently). Not good 
behaviour!
- The default stack size remains unchanged.
- It does not limit the size of the initial thread's stack. The initial 
patch I wrote implements this by inserting a guard page, but it seems 
pointless to do so when we don't have StackOverflowException.

Another issue is that code running in our interpreter currently uses 
substantially more stack than on other implementations, or with gcj 
native code (I suspect this is due largely to libffi's stack allocated 
closures). This means that interpreted code, given the same stack size, 
is more likely to hit the stack limit on libgcj than in other 
implementations.

Despite these issues, the ability to control the stack size is necessary 
for large applications such as Jonas which can quickly run out of 
address space when spawning many threads on a 32-bit machine. I plan to 
post the more extensive (but problematic) implementation soon, but it 
will still need some work before it can be committed.

I'm checking this in to both the 4.0 branch and trunk.

Bryce


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