linux-user: rt_sigprocmask, check read perms first

linux-user: rt_sigprocmask, check read perms first

Linux kernel does it this way (checks read permission before validating `how`)
and the latest version of ABSL's `AddressIsReadable()` depends on this
behavior.

c.f.  https://github.com/torvalds/linux/blob/9539ba4308ad5bdca6cb41c7b73cbb9f796dcdd7/kernel/signal.c#L3147
Reviewed-by: Patrick Venture <venture@google.com>
Signed-off-by: Shu-Chun Weng <scw@google.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Patrick Venture <venture@google.com>
Message-Id: <20220126212559.1936290-2-venture@google.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>