alpha-linux-user: Fix sigprocmask.

Alpha passes oldset by value in a register, and returns the newset
as the return value; as compared to the standard implementation in
which both are passed by reference.  This requires being able to
distinguish negative return values that are not errors.  Do this in
the same way as the Alpha Linux kernel, by storing a zero in V0 in
the implementation of the syscall.

At the same time, fix a think-o in the regular sigprocmask path in
which we passed the target, rather than the host, HOW value.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
