linux-user/syscall.c: fix missed flag for shared memory in open_self_maps

linux-user/syscall.c: fix missed flag for shared memory in open_self_maps

The possible variants for region type in /proc/self/maps are either
private "p" or shared "s". In the current implementation,
we mark shared regions as "-". It could break memory mapping parsers
such as included into ASan/HWASan sanitizers.

Fixes: 01ef6b9e4e4e ("linux-user: factor out reading of /proc/self/maps")
Signed-off-by: Andrey Kazmin <a.kazmin@partner.samsung.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20211227125048.22610-1-a.kazmin@partner.samsung.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>