linux-user: Emulate the Anonymous: keyword in /proc/self/smaps

linux-user: Emulate the Anonymous: keyword in /proc/self/smaps

Core dumps produced by gdb's gcore when connected to qemu's gdbstub
lack stack. The reason is that gdb includes only anonymous memory in
core dumps, which is distinguished by a non-0 Anonymous: value.

Consider the mappings with PAGE_ANON fully anonymous, and the mappings
without it fully non-anonymous.

Tested-by: Helge Deller <deller@gmx.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
[rth: Update for open_self_maps_* rewrite]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>