[sanitizer] [dfsan] Unify aarch64 mapping

This patch changes the DFSan instrumentation for aarch64 to instead
of using fixes application mask defined by SANITIZER_AARCH64_VMA
to read the application shadow mask value from compiler-rt. The value
is initialized based on runtime VAM detection.

Along with this patch a compiler-rt one will also be added to export
the shadow mask variable.

llvm-svn: 254196