linux-user: Update TASK_UNMAPPED_BASE for aarch64

linux-user: Update TASK_UNMAPPED_BASE for aarch64

This change updates TASK_UNMAPPED_BASE (the base address for guest programs) for aarch64. It is needed to allow qemu to work with Thread Sanitizer (TSan), which has specific boundary definitions for memory mappings on different platforms:
https://github.com/llvm/llvm-project/blob/master/compiler-rt/lib/tsan/rtl/tsan_platform.h

Signed-off-by: Lirong Yuan <[email protected]>
Reviewed-by: Laurent Vivier <[email protected]>
Message-Id: <20200313002813[email protected]>
Signed-off-by: Laurent Vivier <[email protected]>