[asan] Enable ASan for AArch64/42-bit VMA

This patch adds support for asan 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.

For 42-bit VMA aarch64 uses SANITIZIER_CAN_USER_ALLOCATOR64.

llvm-svn: 245596