[PATCH] [Sanitizer] Update GetMaxVirtualAddress for PowerPC64 Linux

[Kostya Serebryany]

I am always reluctant to support any old OS (and > 2+ years is old) unless there is a specific request and someone is willing to do the support work. 
My team currently does't test on PowerPC at all. So, the simplest solution (i.e. only support 46-bit and assert at run-time if the OS is old) seems best for me.

http://reviews.llvm.org/D6082