mips: default VKI_SHMLBA to the MIPS Linux kernel value SHMLBA

This change removes option to define shared-memory-alignment for MIPS,
instead default value (0x40000) from MIPS Linux kernel will be used.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13389 a5019735-40e9-0310-863c-91ae7b9d1cf9