[asan] initialize kHighMemEnd at startup (instead of at compile time) to simplify further changes for various address space layouts. Fix asan_allocator2 for PowerPC (tested on 44-bit address space)

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@173260 91177308-0d34-0410-b5e6-96231b3b80d8