[sanitizer] Add MSan support for AArch64

This patch adds support for msan on aarch64-linux for both 39 and
42-bit VMA.  The support is enabled by defining the
SANITIZER_AARCH64_VMA compiler flag to either 39 or 42 at build time
for both clang/llvm and compiler-rt.  The default VMA is 39 bits.

llvm-svn: 247807