mips32: another VG_(am_get_advisory) needs non-single-page-size adjustment

Another mmap issue in which another VG_(am_get_advisory) needs adjustment
wrapper for cases when (VKI_SHMLBA > VKI_PAGE_SIZE) and argument is
VKI_MAP_SHARED.

Fix by DejanJ for Bug #320057.
Issue and the test case by Vasile Floroiu.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13450 a5019735-40e9-0310-863c-91ae7b9d1cf9