[sanitizer] Split GetMaxVirtualAddress into separate Linux version and Mac version [NFC]

The logic in GetMaxVirtualAddress is already pretty complex, and I want to get rid of the hardcoded value for iOS/AArch64, which would need adding more Darwin-specific code, so let's split the implementation into sanitizer_linux.cc and sanitizer_mac.cc files. NFC.

Differential Revision: https://reviews.llvm.org/D35031

llvm-svn: 307281