[TESTCASE] Minimized testcase for AltiVec segfault

[Kumar Gala]

>> VRSAVE is useful for ABIs which allocate registers in order (ie MacOS)
>> such that one can mark the last used register and save/restore to that
>> point.  Otherwise it seems rather painful to manage/update vrsave in 
>> one's
>> code (for SVR4 style register allocation).
>
> ahh, that wasn't clear from the specs

I'm not sure why its not made more clear.  My guess is that the guys who 
came up with it knew about the register allocation in-order on MacOS and 
the Apple guys know about it and took it for granted.

>> If GCC is managing VRSAVE when -mabi=altivec then I think there should 
>> be
>> another flag to disable the use of VRSAVE.  It tends to be a waste of 
>> time
>> to manage as well as hard to use for determining which regs to save by 
>> the
>> context code.
>
> in which case... you are correct.  we should add a flag to not set 
> vrsave.

That would be good (on LinuxPPC the default should be for vrsave to be 
disabled).  I am not sure how register allocation is handled on 
darwin/MacOS-X ABI.  Maybe one of the Apple guys can pipe up, Stan?  My 
guess would be that it does not differ much from how it is handled on 
MacOS.  (I could be wrong).

- kumar