target/hppa: Implement space register hashing for 64-bit HP-UX

The Linux kernel turns space-register hashing off unconditionally at
bootup.  That code was provided by HP at the beginning of the PA-RISC
Linux porting effort, and I don't know why it was decided then why Linux
should not use space register hashing.
32-bit HP-UX versions seem to not use space register hashing either.

But for 64-bit HP-UX versions, Sven Schnelle noticed that space register
hashing needs to be enabled and is required, otherwise the HP-UX kernel
will crash badly.

On 64-bit CPUs space register hashing is controlled by a bit in diagnose
register %dr2.  Since we want to support Linux and 32- and 64-bit HP-UX,
we need to fully emulate the diagnose registers and handle specifically
the bit in %dr2.

This patch adds the code to calculate the gva memory mask based on the
space-register hashing bit in %dr2 and the PSW_W (64-bit) flag.
The value is cached in the gva_offset_mask variable in CPUArchState
and recalculated at every modification of the CPU PSW or %dr2.

Signed-off-by: Helge Deller <deller@gmx.de>
Suggested-by: Sven Schnelle <svens@stackframe.org>
Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

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