[dfsan] Enable dfsan for aarch64

This patch enable DFSan for AArch64 (39-bit VMA).  All tests are passing
but:

 * test/dfsan/custom.cc

Due an invalid access in dl_iterate_phdr instrumentation (commenting out
this function make the testcase to pass).  The test is XFAIL for aarch64
for now.

llvm-svn: 243688