linux-user/x86_64: Handle the vsyscall page in open_self_maps_{2,4}

linux-user/x86_64: Handle the vsyscall page in open_self_maps_{2,4}

This is the only case in which we expect to have no host memory backing
for a guest memory page, because in general linux user processes cannot
map any pages in the top half of the 64-bit address space.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2170
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>