mips64: Correct the value for the VG_MIN_MALLOC_SZB

The VG_MIN_MALLOC_SZB was incorrectly defined for MIPS64.
The incorrect value was 8 and the correct value is 16.
Fixes massif/tests/big-alloc for MIPS64.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13455 a5019735-40e9-0310-863c-91ae7b9d1cf9