[PATCH] [compiler-rt] fix and simplify GetMaxVirtualAddress for ppc64*

[Kostya Serebryany]

I would add a test similar to test/asan/TestCases/debug_mapping.cc that will use ASAN_OPTIONS=verbosity=1 
to check that the mapping is what's expected on the given system. 

The test will need the REQUIRES: clause similar to the one below, but for powerpc
// REQUIRES: x86_64-supported-target 
( I am not sure what exactly that should be. Alexey?)

http://reviews.llvm.org/D6044