[dfsan] Enable DFSan for AArch64/42-bit VMA

This patch adds support for dfsan on aarch64-linux with 42-bit VMA
(current default config for 64K pagesize kernels).  The support is
enabled by defining the SANITIZER_AARCH64_VMA to 42 at build time
for both clang/llvm and compiler-rt.  The default VMA is 39 bits.

llvm-svn: 245841