[PATCH] Allow target to specify prefix for labels

[Matt Arsenault]

>>! In D6467#4, @rafael wrote:
> This is probably OK.
> 
> My only question is how long do you expect this to be dead code? That is, how long until we have a test with HSAIL showing the @ and & difference?

The plan is to upstream the HSAIL target as soon as possible, which is once I've finished fixing a long list of problems with it. My hope is this will take < 3 months

http://reviews.llvm.org/D6467