target/hppa: fix PSW Q bit behaviour to match hardware

target/hppa: fix PSW Q bit behaviour to match hardware

PA-RISC specification says: "Setting the PSW Q-bit, PSW{28}, to 1
with this instruction, if it was not already 1, is an undefined
operation." However, at least HP-UX 10.20 sets the Q bit from 0 to 1
with the SSM instruction. Tested this both on HP9000/712 and
HP9000/785/C3750, both machines set the Q bit from 0 to 1 without
exception. This makes HP-UX 10.20 progress a little bit further.

Signed-off-by: Sven Schnelle <svens@stackframe.org>
Message-Id: <20190129191402.29539-1-svens@stackframe.org>
[rth: Add a comment to the code as well.]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>