[AArch64] Speed-up leak and address sanitizers on AArch64 for 48-bit VMA

This patch fixes https://github.com/google/sanitizers/issues/703
On a Graviton-A1 aarch64 machine with 48-bit VMA,
the time spent in LSan and ASan reduced from 2.5s to 0.01s when running

clang -fsanitize=leak compiler-rt/test/lsan/TestCases/sanity_check_pure_c.c && time ./a.out
clang -fsanitize=address compiler-rt/test/lsan/TestCases/sanity_check_pure_c.c && time ./a.out

With this patch, LSan and ASan create both the 32 and 64 allocators and select
at run time between the two allocators following a global variable that is
initialized at init time to whether the allocator64 can be used in the virtual
address space.

Differential Revision: https://reviews.llvm.org/D60243

llvm-svn: 369441