Gracefully handle PIE binary with Symbion

[degrigis]

The new ConcreteBackerMixin for Symbion introduced a sneaky bug for PIE binaries.
When a base_addr is not specified in the main_opts for PIE binaries, the concrete_target will redirect reads to the concrete process using as base the 0x400000 address. This is of course failing.
The current workaround is to simply specify the base address of the binary from the beginning:

BASE_ADDRESS = 0x0000555555554000
self.avatar_gdb = AvatarGDBConcreteTarget(avatar2.archs.x86.X86_64, GDB_SERVER_IP, GDB_SERVER_PORT)
p = angr.Project(binary_x64, concrete_target=self.avatar_gdb, use_sim_procedures=True,
                           page_size=0x1000, main_opts={'base_addr': BASE_ADDRESS})

However, a cleaner solution would try to force the memory mapping synchronization before doing any read.