linux-user: sparc64: Use correct target SHMLBA in shmat()

linux-user: sparc64: Use correct target SHMLBA in shmat()

In commit 40df8c0c0722 support was added for target-specific
handling of SHMLBA. Unfortunately the sparc64-specific part
of the change got lost somewhere between the patch being
posted to the list and going into master:
 http://patchwork.ozlabs.org/patch/646980/
 http://patchwork.ozlabs.org/patch/673339/

Add the accidentally-dropped code.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>