[compiler-rt] [dfsan] Unify aarch64 mapping

    
This patch reorganize the platform specific mapping information to
export the application mask on a external variable. This exported
variable will be used by intrumentation phase to create code to be
used on architecture with multiple VMA range.
    
The patch creates a new header, dfsan_platform.h, and move all the
mapping information and also create function accessors to the
mapping value.

Also for aarch64 it initialize application exported mask to the 
value based on runtime VMA detection.

llvm-svn: 254197