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 <yuanzi@google.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200313002813.3857-1-yuanzi@google.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

linux-user/mmap.c

@@ -184,7 +184,11 @@ static int mmap_frag(abi_ulong real_start,
 }
 
 #if HOST_LONG_BITS == 64 && TARGET_ABI_BITS == 64
+#ifdef TARGET_AARCH64
+# define TASK_UNMAPPED_BASE  0x5500000000
+#else
 # define TASK_UNMAPPED_BASE  (1ul << 38)
+#endif
 #else
 # define TASK_UNMAPPED_BASE  0x40000000
 #endif